Add getPoolSyncCommitteeSignatures route

[dapplion]

• No endpoint to get sync committee contributions from pool #186 noted missing route to retrieve SyncCommitteeSignature from the pool

There hasn't been much demand for it, but there's no good reason to omit this route on purpose so adding for completeness

Closes #186

[rolfyone]

There doesn't seem to be much interest in adding this without a use-case..

[tersec]

"no good reason to omit" isn't a reason to add. Oppose.

[rolfyone]

closing, as no real takers for wanting this.