Add provider name to BuildSystem

[gitfool]

Add provider name to BuildSystem to simplify build info introspection.

This can be done with reflection to keep it simple and dynamic, as I do in Cake.Dungeon/scripts/builder.cake, so the build info introspection can easily filter the BuildSystem properties to just the relevant provider.

For example, this is what my build log is currently generating on Azure Pipelines:

2019-01-04T20:00:07.5090544Z ========================================
2019-01-04T20:00:07.5091011Z Info
2019-01-04T20:00:07.5091277Z ========================================
2019-01-04T20:00:07.5472444Z 
2019-01-04T20:00:07.5480136Z BuildSystem.TFBuild.IsRunningOnVSTS                                 true
2019-01-04T20:00:07.5480501Z BuildSystem.TFBuild.IsRunningOnTFS                                  false
2019-01-04T20:00:07.5480670Z BuildSystem.TFBuild.Environment.Repository.Branch                   master
2019-01-04T20:00:07.5480852Z BuildSystem.TFBuild.Environment.Repository.SourceVersion            93a89d64a2efc6328ac4ddf46af8b220229235cf
2019-01-04T20:00:07.5481036Z BuildSystem.TFBuild.Environment.Repository.Shelveset                "
2019-01-04T20:00:07.5481221Z BuildSystem.TFBuild.Environment.Repository.RepoName                 gitfool/Cake.Dungeon
2019-01-04T20:00:07.5484630Z BuildSystem.TFBuild.Environment.BuildDefinition.Id                  1
2019-01-04T20:00:07.5485136Z BuildSystem.TFBuild.Environment.BuildDefinition.Name                gitfool.Cake.Dungeon
2019-01-04T20:00:07.5485687Z BuildSystem.TFBuild.Environment.BuildDefinition.Version             1
2019-01-04T20:00:07.5486019Z BuildSystem.TFBuild.Environment.Build.Id                            6
2019-01-04T20:00:07.5486344Z BuildSystem.TFBuild.Environment.Build.Number                        20190104.2
2019-01-04T20:00:07.5486699Z BuildSystem.TFBuild.Environment.Build.Uri                           vstfs:///Build/Build/6
2019-01-04T20:00:07.5487052Z BuildSystem.TFBuild.Environment.Build.QueuedBy                      GitHub
2019-01-04T20:00:07.5487311Z BuildSystem.TFBuild.Environment.Build.RequestedFor                  GitHub
2019-01-04T20:00:07.5487613Z BuildSystem.TFBuild.Environment.Build.RequestedForEmail             "
2019-01-04T20:00:07.5487923Z BuildSystem.TFBuild.Environment.TeamProject.Name                    Cake.Dungeon
2019-01-04T20:00:07.5489524Z BuildSystem.TFBuild.Environment.TeamProject.Id                      76668ef0-590e-4f12-8b2f-47912845203c
2019-01-04T20:00:07.5495431Z BuildSystem.TFBuild.Environment.TeamProject.CollectionUri           https://dev.azure.com/gitfool/
2019-01-04T20:00:07.5497323Z BuildSystem.TFBuild.Environment.Agent.BuildDirectory                /home/vsts/work/1
2019-01-04T20:00:07.5497783Z BuildSystem.TFBuild.Environment.Agent.HomeDirectory                 /home/vsts/agents/2.144.0
2019-01-04T20:00:07.5498281Z BuildSystem.TFBuild.Environment.Agent.WorkingDirectory              /home/vsts/work
2019-01-04T20:00:07.5499407Z BuildSystem.TFBuild.Environment.Agent.Id                            5
2019-01-04T20:00:07.5499746Z BuildSystem.TFBuild.Environment.Agent.Name                          Hosted Agent
2019-01-04T20:00:07.5545168Z BuildSystem.TFBuild.Environment.Agent.MachineName                   "
2019-01-04T20:00:07.5547479Z BuildSystem.TFBuild.Environment.Agent.IsHosted                      true
2019-01-04T20:00:07.5547807Z 
2019-01-04T20:00:07.5548309Z Context.Environment.WorkingDirectory                                /home/vsts/work/1/s
2019-01-04T20:00:07.5549395Z Context.Environment.ApplicationRoot                                 /home/vsts/work/1/s/tools/Cake
2019-01-04T20:00:07.5549704Z Context.Environment.Platform.Is64Bit                                true
2019-01-04T20:00:07.5549980Z Context.Environment.Runtime.BuiltFramework.Identifier               .NETFramework
2019-01-04T20:00:07.5550263Z Context.Environment.Runtime.BuiltFramework.Version.Major            4
2019-01-04T20:00:07.5550502Z Context.Environment.Runtime.BuiltFramework.Version.Minor            6
2019-01-04T20:00:07.5550778Z Context.Environment.Runtime.BuiltFramework.Version.Build            1
2019-01-04T20:00:07.5551557Z Context.Environment.Runtime.BuiltFramework.Version.Revision         -1
2019-01-04T20:00:07.5552139Z Context.Environment.Runtime.BuiltFramework.Version.MajorRevision    -1
2019-01-04T20:00:07.5552801Z Context.Environment.Runtime.BuiltFramework.Version.MinorRevision    -1
2019-01-04T20:00:07.5553029Z Context.Environment.Runtime.BuiltFramework.Profile                  "
2019-01-04T20:00:07.5553300Z Context.Environment.Runtime.BuiltFramework.FullName                 .NETFramework,Version=v4.6.1
2019-01-04T20:00:07.5553553Z Context.Environment.Runtime.CakeVersion.Major                       0
2019-01-04T20:00:07.5553793Z Context.Environment.Runtime.CakeVersion.Minor                       31
2019-01-04T20:00:07.5553979Z Context.Environment.Runtime.CakeVersion.Build                       0
2019-01-04T20:00:07.5554276Z Context.Environment.Runtime.CakeVersion.Revision                    0
2019-01-04T20:00:07.5554544Z Context.Environment.Runtime.CakeVersion.MajorRevision               0
2019-01-04T20:00:07.5554784Z Context.Environment.Runtime.CakeVersion.MinorRevision               0
2019-01-04T20:00:07.5554976Z Context.Environment.Runtime.IsCoreClr                               false
2019-01-04T20:00:07.5555215Z Context.Arguments.Arguments                                         {}

This can help diagnose issues with your Cake build scripts.