Extend relayer for activity rollup and L2+

[cryptoAtwill]

With the integration of L2+ and validator rewarder, it is helpful to extend existing relayer functionalities with:

Watching multiple subnets at once

Enabling relayer to submit bottom up checkpoints for multiple subnets in one instance. This reduces the number of relayers required to maintain, say an L3.

Processing summaries on all subnets

Adding a new flag --process-summaries to activate processing summaries on all subnets.
Enabling this mode makes the relayer watch for ActivitySummaryCommitted events, and stores the summaries in a database.
It then tracks which summaries have been committed to the root (right now we only support submitting to the L1), to chase after those and present them via SubnetActor#submitSummary in order to trigger reward payout.