Add ability to specify "full" scheme actions with TargetScheme

[brentleyjones]

FYI: I'll break this in a couple PRs (code cleanup, adding parallelizeBuild, adding some test fixtures, and finally adding the full scheme support to target) if everything looks good.

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This PR adds support for specifying full scheme configuration to target: Target: scheme:. If any of the actions are specified (e.g. build, test, profile) then that block will be used the same as a top level scheme declaration. For any steps not specified they will use the existing helper/defaulting logic.

This allows one to do something like this:

targets:
  App:
    scheme:
      build:
        targets:
          App: all
      run:
        commandLineArguments:
          "-enable-perf-logging": false
        environmentVariables:
          - variable: TSAN_OPTIONS
            value: suppressions=$(SRCROOT)/tsan-suppressions.txt
            isEnabled: true
      test:
        targets:
         - Apps Tests
        gatherCoverageData: true
      profile:
        config: Profile
      archive:
        config: Release

This should help address issue like #656.

[yonaskolb]

Looks like the default changed

[yonaskolb]

Never-mind looks like you set it explicitly down below

[yonaskolb]

@brentleyjones love this! 👏

[brentleyjones]

Some things came up, but I'll get to this soon.

[brentleyjones]

If this is still desired, someone else will have to re-submit it. I no longer use XcodeGen.