Prepare v2 release with breaking changes

[ErikPelli]

net.Dial is deprecated.
Replace its usages with net.DialContext.

[ErikPelli]

This will be done in a v2 branch, because it's a breaking change

[ErikPelli]

• Replace Dial with DialContext
• Review public exposed functions and remove the deprecated ones
• Add a TLS interface to allow other TLS libraries implementations (e.g. uTLS)
• Add Warn, Info and Error to external logger interface to log different stuff based on the type of error
• Remove custom library context and replace it with context.Context (goproxy2)
• Remove custom legacy transport.go file and replace it with the go standard library

[ErikPelli]

• Use only one error handler instead of a lot of user specified functions, and use an enum as the first parameter, to specify the error source. Delete ctx.Error since the error will be handled directly in this handler.

[ErikPelli]

• Refactor HTTP/2 code

[ErikPelli]

• Check abourget fork to see if there are some useful refactorings that can be integrated