changing gaslimit to validator registration

[james-prysm]

What type of PR is this?

Feature

related to #10593
What does this PR do? Why is it needed?
This feature brings a way to define validator registration ( builder API) usage by validator public key in the proposer settings file and is intended to mirror teku. if the validator registration portion is filled then it will be used to call the submit validator registration API.

• This introduces a breaking change for individuals who are using the gas limit feature. : this will wrap gas limit with another object that includes an optional enable field.

• update proposer settings file to include a validator registration object instead of just gas_limit

{
  "proposer_config": {
    "0xa057816155ad77931185101128655c0191bd0214c201ca48ed887f6c4c6adf334070efcd75140eada5ac83a92506dd7a": {
      "fee_recipient": "0x50155530FCE8a85ec7055A5F8b2bE214B3DaeFd3",
      "validator_registration": {
        "enable": true,
        "gas_limit": 30000000
      }
    },
   
  "default_config": {
    "fee_recipient": "0x6e35733c5af9B61374A128e6F85f553aF09ff89A",
    "validator_registration": {
      "enable": true,
      "gas_limit": 30000000
    }
  }
}

• new --enable-validator-registration flag which can be used with the --suggested-fee-recipient flag on the validator client to enable the periodic calling validator registration API for MEV builders

• updated docs (Fee Recipient: Validator Registration documentation#494)