change: ETCM-12419 decouple sidechain-rpc from slots

[AmbientTea]

Description

Removes dependency on slots crates from pallet-sidechain-rpc (sidechain_getStatus) by:

• Adding a new runtime api sp_sidechain::GetEpochDurationApi and computing current/next PC epoch using it instead of slots
• Removing sidechain.slot from the response by default
• Adding a feature flag legacy-compat which adds the ability to read epoch duration / slots per epoch using raw calls to SlotApi and re-adds the sidechain.slot field as optional

The backward compatibility aspect of this PR is probably not really needed here, but we'll need to do the same in other parts of the codebase that are more important (eg. committee selection IDP needs access to epoch duration data, which for old blocks will need to be computed based on slot configuration exposed viaSlotApi).

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• The size limit of 400 LOC isn't needlessly exceeded
• The PR refers to a JIRA ticket (if one exists)
• New tests are added if needed and existing tests are updated.
• New code is documented and existing documentation is updated.
• Relevant logging and metrics added
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

[kpinter-iohk]

minor comment