Suggestions for fee recipieint changes

[paulhauner]

Issue Addressed

NA

Proposed Changes

Adds some suggested changes as a part of a review on sigp#2924:

• Renames the --fee-recipient flag to --suggested-fee-recipient for consistency with engine API.
• Adds the suggested_fee_recipient to the validator_definitions.yml file.
• Adds documentation about the fee recipient.
• The now VC polls the prepare endpoing on the BN once per slot.
• The VC will immediately poll the prepare endpoint when the PreparationService starts.
• Removes the hard-coded default fee-recipient from the BN. It was marked with a TODO and needed to be removed.

Additional Info

NA