Add fork-agnostic api/v1 types (api/v1/all)

[pk910]

Add fork-agnostic api/v1 types (api/v1/all)

Motivation

The spec/all package provides fork-agnostic union types for consensus spec
containers, but the api/v1 wire types (block contents, blinded blocks) only
exist as per-fork packages. Consumers that handle multiple forks — builders,
relays, tooling — have to hardcode a fork package (apiv1fulu,
apiv1electra, ...) and duplicate a version switch at every call site.

This PR adds api/v1/all: fork-agnostic counterparts for the api/v1 wire
types, following the spec/all conventions (version-multiplexed views,
dynamic-ssz codecs, JSON/YAML delegation to the per-fork wire shapes).

Types

Type	Forks	Notes
BlockContents / SignedBlockContents	Deneb, Electra, Fulu	pre-Deneb submits bare blocks; other versions rejected
BlindedBeaconBlockBody / BlindedBeaconBlock / SignedBlindedBeaconBlock	Bellatrix → Electra (+ Fulu on the Electra schema)	mirrors spec/all's Electra/Fulu schema sharing

Each type carries a Version field and spec/all component fields
(*all.SignedBeaconBlock, *all.ExecutionPayloadHeader, ...), and implements
the full spec/all codec surface:

• dynamic-ssz view codecs (MarshalSSZDyn / UnmarshalSSZDyn /
  SizeSSZDyn / HashTreeRootWithDyn) plus the fastssz-compatible wrappers,
  generated via dynssz-gen (view-only mode, like spec/all)
• JSON/YAML marshaling that delegates to the per-fork wire type, so the bytes
  are identical to the fork-specific packages'
• ToView / FromView for converting to/from the per-fork types
• ToVersioned / FromVersioned bridging to api.VersionedProposal,
  api.VersionedSignedProposal, api.VersionedBlindedBeaconBlock,
  api.VersionedSignedBlindedBeaconBlock

Additionally, ProposalFromSignedBlock(*all.SignedBeaconBlock) maps a
post-Gloas bare block into api.VersionedSignedProposal (Gloas/Heze). From
Gloas onwards blocks are submitted without a block-contents wrapper — EIP-7732
separates the execution payload and blobs from the block — so the bare-block
mapping lives here as a helper rather than as a SignedBlockContents version.

Fix included

spec/all.ExecutionPayloadHeader.viewType only accepted
Bellatrix/Capella/Deneb, while ExecutionPayload already mapped
Electra/Fulu → Deneb. An Electra- or Fulu-versioned agnostic header failed to
encode. The multiplexers now match.

Wire compatibility

JSON/SSZ marshaling delegates to the same per-fork view the concrete types
use, so compatibility holds by construction — and is pinned by tests:

• Test*JSONWireCompat / Test*SSZWireCompat: byte-identical JSON and SSZ
  (and equal hash tree roots) versus the fork-specific type built from the
  same data, per version
• Test*ToVersioned: the populated Versioned* field is the exact per-fork
  view; for proposals additionally Blinded == false and byte-identical
  encoding of the field the HTTP client actually submits
• TestSignedBlockContentsFromVersionedBlinded: blinded proposals are
  rejected rather than misread
• Test*ViewRoundtrip, Test*UnsupportedVersion, version-propagation checks

Notes

• One helper (propagateVersion) re-implements spec/all's unexported
  populateVersion cascade via reflection, since it is unreachable
  cross-package; behavior is asserted by the round-trip tests.
• Generated *_ssz.go files are not gofmt-formatted, matching the existing
  generated files in spec/all.