bridge: migrate cmd/ to cobra

[leoluk]

Stack from ghstack:

• solana: add uncommitted Cargo.lock files #68 solana: add uncommitted Cargo.lock files
• bridge: migrate cmd/ to cobra #67 bridge: migrate cmd/ to cobra
• Remove outdated TODO comments #66 Remove outdated TODO comments
• bridge: refactor p2p logic into pkg/p2p #65 bridge: refactor p2p logic into pkg/p2p
• bridge: remove all supervisor.SignalHealthy calls #64 bridge: remove all supervisor.SignalHealthy calls
• bridge: refactor processor logic into pkg/processor #63 bridge: refactor processor logic into pkg/processor
• node.proto stub and dependencies #62 node.proto stub and dependencies
• devnet: use real account and nonce for send-lockups.js #61 devnet: use real account and nonce for send-lockups.js
• bridge: bypass p2p for our own signatures #60 bridge: bypass p2p for our own signatures
• bridge: verify LockupObservation signature #59 bridge: verify LockupObservation signature
• bridge: correctly calculate 2/3+ majority #58 bridge: correctly calculate 2/3+ majority
• IDE helper scripts for logs and lockups #57 IDE helper scripts for logs and lockups

Reviewer note: Does not touch any of the business logic. Avoided
renaming files whereever possible to make it easier to spot differences.

Verbatim migration, in a future CL, we could replace some of the
flag validation code with cobra features and eliminate the global vars.

Moved the dlv tool definition out of the way for the top-level wrapper.

tools/bin/cobra is a helper utility that generates boilerplate
(we slightly deviate from their default scheme by having guardiand
in a separate package, rather than stuffing everything into cmd/)