feat: break saveRecord up into legacy operations

[runspired]

(2/4ths of) the literal final step to complete #6166 💚

This PR:

• saveRecord as request op

Next PRs:

• [-] findBelongsTo as request op
• [-] findHasMany as request op

To Go (for full RequestManager support): (not this PR)

• [-] enforce immutability in dev
• [-] handle error documents in cache
• [-] handle meta documents in cache
• [-] additional test coverage for Store.request, errors thrown during request
• [-] additional test coverage for cache.peek / cache.put / cache.peekRequest
• [-] update docs for RequestManager.useCache
• [-] implement new Collection mode to properly reflect ResourceDocument
• [-] ensure single-resource-document requests are notified when data changes
• [-] determine Cache "mutability" of CollectionResourceDocument
• [-] add tests for enhancing context.request in RequestManager before calling next

Other Notes:

• clarify intended Error handling mechanisms/propagation
• update RFC for RequestManager.useCache
• update RFC for lifetimes.isSoftExpired/isHardExpired
• update RFC for rename data => content