Subscribe only mode, port prometheus metrics and jemalloc from v0.2-triton

[leafaar]

Adds a subscribe-only mode and brings over the metrics work from v0.2-triton so we stop losing it now that releases are built from master.

• new subscribe-only subcommand: does auth and heartbeat against the block engine but doesn't bind a socket or forward anything, useful when another process is already listening on the dest port
• merged latest master to pick up the goreleaser gen2 config
• cherry picked prometheus support from v0.2-triton (str-57 plus the extra metrics from str-57: add prometheus support #8 and 00004: use jemalloc and fix recv_mmmsg batch size hist metric #9), exposed via --prometheus-bind-addr, disabled if not set
• jemalloc as the global allocator, same as the old 0004 build

Skipped the old triton multicast patches since master already has the upstream multicast implementation and the triton variant was reverted there.

clippy with -D warnings passes. The two deshred test failures locally are just the lfs fixtures not being pulled, CI checks out with lfs so they pass there.