Add internal/mux

[komuw]

• This internal muxer contains the bulk of implementation and the exported muxer forwards to it.

• This is done so that we can be able to add methods to the muxer that are needed by ong but aren't necessary for external users.

• We also add one internal muxer method AddRoute. This can be used to add a route to an already existing muxer.
  It is used by ong/server to add a route to handle ACME requests.
  It will also be used in future to add a route for handling pprof requests(pprof server should also be available over https with redirection #366)

• Updates: pprof server should also be available over https with redirection #366

[codecov-commenter]

Codecov Report

Patch coverage: 77.39% and project coverage change: -0.38% ⚠️

Comparison is base (c1f3b86) 74.64% compared to head (b5e05af) 74.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
- Coverage   74.64%   74.27%   -0.38%     
==========================================
  Files          44       46       +2     
  Lines        5270     5318      +48     
==========================================
+ Hits         3934     3950      +16     
- Misses       1068     1099      +31     
- Partials      268      269       +1     

Files Changed	Coverage Δ
mux/mux.go	0.00% <0.00%> (-72.73%)	⬇️
mux/route.go	0.00% <0.00%> (-90.41%)	⬇️
server/pprof.go	87.50% <ø> (ø)
internal/mx/mx.go	65.33% <65.33%> (ø)
server/server.go	76.22% <81.25%> (+0.32%)	⬆️
internal/mx/route.go	90.97% <90.97%> (ø)
internal/acme/acme.go	74.48% <100.00%> (ø)

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