Implement keymanager API per-validator builder configuration

[tosynthegeek]

Implement the per-key builder configuration keymanager endpoints defined in ethereum/keymanager-APIs#88. This is the remaining piece of Gloas builder API work by #9757.

Scope

• Add the Key Manager API endpoints and types required for per-validator builder configuration, following the current API shape in spec PR
• Extend the builder_types added in Gloas: external builder API (payload bid selection and block production) #9757
• Support configuring builders for an individual validator public key.
• Support retrieving the configured builder configuration for a validator.
• Integrate the configuration with existing validator/builder configuration.
• Ensure the proposer/builder flow can resolve the builder configuration for the validator proposing a block.
• Add tests covering the new functionality.
• Keep the implementation aligned with the Key Manager API as spec PR evolves.

References

Spec PR: ethereum/keymanager-APIs#88
Builder API Issue: #9590
Builder API PR: #9757