Instance config with stricter offset is compatible

[antiguru]

Introduce InstanceConfig::compatible_with to check if two configurations
are compatible. Two configurations are considered compatible iff the
logging configuration is equal and the expiration offset of the other
configuration is stricter than or equal to the current one.

Addresses a concern with replica expiration that didn't allow us to
strengthen the value without risking replica restarts in case of envd
restarts. This doesn't solve the problem of relaxing the expiration, which
would still cause replica restarts -- there doesn't seem to be a good
alternative because the replicas aren't actually compatible.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[teskje]

LGTM!