Interface for ptrs pluggable transports crate

[jmwample]

this pr adds an interface (a set of traits) that a pluggable transport can implement to be generically interoperable.

This is not a final form commit, but more of an MVP. For example there is still a question of how the set local IPv4 and IPv6 addresses get applied. Also the exact set of mutator methods that the {Client/Server}Transport traits should include.

For now this is good enough.

The PR includes an example Passthrough transport that simply returns the original stream/connection as a mock transport to test things like boxed futures and composing transports.

[codecov]

Codecov Report

Attention: Patch coverage is 54.57481% with 422 lines in your changes are missing coverage. Please review.

Project coverage is 69.65%. Comparing base (b4dcf87) to head (75cbb7c).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/ptrs/src/helpers.rs	0.00%	172 Missing ⚠️
crates/ptrs/src/passthrough.rs	76.59%	24 Missing and 130 partials ⚠️
crates/obfs4/src/pt.rs	0.00%	54 Missing ⚠️
crates/obfs4/src/obfs4/client.rs	7.69%	24 Missing ⚠️
crates/obfs4/src/obfs4/server.rs	7.69%	11 Missing and 1 partial ⚠️
crates/ptrs/src/lib.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
- Coverage   73.29%   69.65%   -3.64%     
==========================================
  Files          34       39       +5     
  Lines        3853     4805     +952     
  Branches     3853     4805     +952     
==========================================
+ Hits         2824     3347     +523     
- Misses        798     1094     +296     
- Partials      231      364     +133     

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