bridge: move initial guardian set fetching to pkg/ethereum/watcher.go

[leoluk]

Stack from ghstack:

• bridge: log submission for cross-submissions to Solana #54 bridge: log submission for cross-submissions to Solana
• bridge: do not resubmit submitted VAAs during aggregation #53 bridge: do not resubmit submitted VAAs during aggregation
• bridge: do not log errors for duplicate VAA submissions #52 bridge: do not log errors for duplicate VAA submissions
• bridge: have all nodes submit VAAs to Solana #51 bridge: have all nodes submit VAAs to Solana
• bridge/pkg/solana: retry VAA submission on transient errors #48 bridge/pkg/solana: retry VAA submission on transient errors
• agent: return gRPC Internal error on submission failure #47 agent: return gRPC Internal error on submission failure
• bridge: move initial guardian set fetching to pkg/ethereum/watcher.go #46 bridge: move initial guardian set fetching to pkg/ethereum/watcher.go
• bridge: propagate panics from runnables #45 bridge: propagate panics from runnables
• bridge: in-place debugging using dlv #44 bridge: in-place debugging using dlv

This removes the special case in the processor. The initial guardian set
is now treated like a regular guardian set update, and the devnet
update check is executed on every update.

Fixes an edge case where processing a guardian set update would fail
with a spurious abi: attempting to unmarshall an empty string while arguments are expected
error, leaving the node in a bad state since restarting ethwatch
wouldn't cause the guardian set to be re-fetched.