feat: Add SSZ response support for validator_balances endpoints

[NikhilSharmaWe]

Closes #793

Adds SSZ (application/octet-stream) response support to GET and POST /eth/v1/beacon/states/{state_id}/validator_balances, per beacon-APIs #622.

Clients opt in via Accept: application/octet-stream. JSON behavior is unchanged.

[NikhilSharmaWe]

Companion snapshot-tests PR: grandinetech/grandine-snapshot-tests#7

@hangleang I kept this PR focused on the API change, so I haven’t bumped the submodule yet. Once grandinetech/grandine-snapshot-tests#7 merges to main, I can push that update here right away.

As I’m new to the project, does this sequence look right, or would you prefer a different order for paired grandine / grandine-snapshot-tests PRs?

[hangleang]

As I’m new to the project, does this sequence look right, or would you prefer a different order for paired grandine / grandine-snapshot-tests PRs?

Just commit with the snapshot tests, make sure the tests is published to remote. I will allow CI to run after that

[NikhilSharmaWe]

As I’m new to the project, does this sequence look right, or would you prefer a different order for paired grandine / grandine-snapshot-tests PRs?

Just commit with the snapshot tests, make sure the tests is published to remote. I will allow CI to run after that

Pushed the submodule bump

[NikhilSharmaWe]

@hangleang applied a fix for the Clippy type_complexity failure - added the missing #[expect(clippy::type_complexity)] on the GET handler and shared helper. Should be good for CI now.

[hangleang]

@povi could you give a yes or no for this PR? it is looks good to me

[povi]

LGTM