Create a standalone tls library that does not require patching the Go standard library

Let's make it separated library instead of being part of Go standard library.
If possible we should keep mechanism allowing to swap sources of tris with Go implementation

The issue does not name files, tests, or an entry point. Start by mapping where the current implementation depends on Go's standard library and clarify the proposed mechanism for swapping sources with the Go implementation. Done means the TLS functionality is a standalone library without patching Go, while preserving the requested substitution mechanism.

Written by the indexing model from the issue text.