COR-232 Feature gate unnecessary dependencies in mirrord-protocol

[DmitryDodzin]

Quality Checklist:

• I have documented new code sufficiently
• I have checked and updated the relevant existing docs in code, including removing outdated material
• I have written user-facing website docs for new features, or opened a (sub)issue to do so
• I have checked and updated existing website docs for changed features
• I have tested this change and know it succeeds and fails as expected
• I have written unit tests or purposefully omitted them
• I have written e2e tests or purposefully omitted them
• I have explained what this PR introduces and why, and linked to relevant context (e.g. linear issues, related PRs,
  documentation)
• I have introduced a short, clear and well-formatted changelog entry

[meowjesty]

Would be nice if we could move these optional impls to separate crates, so we can feature gate the whole crate, instead of doing it with a bunch of cfgs in the code.