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) (Refactor context to use golang's context #227)
• 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

[ErikPelli]

• Completely separate ReqHandler from RespHandler and group requests and resp with the same condition

[ErikPelli]

• Once we have a Go standard context, use different keys for different user data (e.g. one key for each ext package)

[elazarl]

That's a great idea, when I started the library there was not standard context IIRC

[ErikPelli]

I'm annotating here the breaking changes that I'm planning to introduce in this hypothetical new version, it's still a WIP.
If you have any other idea, please write it here!

[ErikPelli]

• Inherit organization name from used certificate

[ErikPelli]

(deleted unrelated comments that were referring a closed issue)

[ErikPelli]

• Try to merge HTTP MITM and TLS MITM actions in order to avoid confusion