fix: validate fee recipient for proposed block

[matthias-wright]

Builds on #191 (which builds on #190).

This addresses #200.

Changes:

• Adds a check for the fee recipient to handle_verify. This check is only performed, if the treasury_address is non-zero. If the treasury_address is set to the zero address, then block proposers may chose any fee recipient.
• Adds unit tests for verifying this.

[evonide]

This appears to verify the fee recipient when treasury mode is nonzero but leaves the zero-treasury/proposer-recipient case unchecked. Should verification require the payload fee recipient to match the expected proposer withdrawal address in that mode, too?

[matthias-wright]

When the treasury address is set to zero, the block proposer may use its own address for the fee recipient (or any other address), so I think this check should cover it.