Modify trillium_server_common::Acceptor to return an Option

[joshtriplett]

This allows an Acceptor to handle some connections entirely internally
and not produce a stream to handle requests from.

[codecov]

Codecov Report

Patch coverage: 56.25% and project coverage change: -0.09% ⚠️

Comparison is base (0853e5d) 45.03% compared to head (d64013c) 44.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
- Coverage   45.03%   44.94%   -0.09%     
==========================================
  Files         156      156              
  Lines        6104     6107       +3     
==========================================
- Hits         2749     2745       -4     
- Misses       3355     3362       +7     

Files Changed	Coverage Δ
native-tls/src/server.rs	50.00% <50.00%> (-7.15%)	⬇️
rustls/src/server.rs	63.15% <50.00%> (-10.18%)	⬇️
server-common/src/config_ext.rs	40.25% <50.00%> (-0.53%)	⬇️
server-common/src/acceptor.rs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[joshtriplett]

Closing this in favor of #377 which uses the suggested approach of returning a dummy Transport after processing a challenge.