Allow customizing the logger middleware source

[funwithcthulhu]

Summary

• Add ?log:Dream.sub_log to Dream.logger, preserving the current dream.logger source by default.
• Expose the same optional argument in the Mirage signature.
• Add unit coverage using an in-memory sub_log to check which log levels the middleware calls.

Closes #413.

Validation

• git diff --cached --check
• opam exec -- dune build src/dream.mli
• opam exec -- dune runtest test/unit (blocked locally: this switch is missing alcotest)
• opam exec -- dune build src/dream.cma src/dream.cmxa (blocked locally: this switch is missing graphql-lwt)
• opam exec -- ocamlformat --inplace ... (not run: project requires ocamlformat 0.25.1, local switch has 0.29.0)