Adds a function that constructs a Merkle proof for a generalized index.
This will be used during light client sync to update light clients with
a new state (see NEXT_SYNC_COMMITTEE_INDEX / FINALIZED_ROOT_INDEX).
* update to Altair as of v1.1.0-alpha.7
* introduce Altair types into attestation pool
* avoid allocating/copying pubkeys excessively in get_next_sync_committee()