gloas blocks rpc limits resizing

[shane-moore]

Changes

• gloas cl blocks are significantly smaller now, so the rpc limits for gloas blocks from inbound responses can be reduced

[shane-moore]

Closing — envelope RPC upstream + block-size resize explicitly rejected upstream.

The envelope RPC scaffolding (most of this PR's diff, inherited from base branch) is upstreamed via sigp#8896 ("Gloas serve envelope rpc", c7055b604).

The headline change — resizing block RPC limits down for GLOAS — was explicitly rejected upstream. rpc_block_limits_by_fork in beacon_node/lighthouse_network/src/rpc/protocol.rs:138-162 keeps Bellatrix-sized max for GLOAS, with a comment added precisely to explain why:

"Gloas blocks no longer contain the execution payload, but we must still accept pre-Gloas blocks during historical sync, so we keep the Bellatrix max as the upper bound."

Upstream considered the same idea and chose the opposite for backfill-sync correctness. Nothing salvageable to upstream from this fork PR.