chore(conductor): avoid large enum variants

[ethanoroshiba]

We should void having large enum variants as described here: https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant

Problem point:

astria/crates/astria-conductor/src/executor/mod.rs

Lines 70 to 90 in 7e00378

	#[derive(Debug, thiserror::Error)]
	pub(crate) enum FirmSendError {
	#[error("executor was configured without firm commitments")]
	NotSet,
	#[error("failed sending blocks to executor")]
	Channel {
	#[from]
	source: mpsc::error::SendError<ReconstructedBlock>,
	},
	}
	#[derive(Debug, thiserror::Error)]
	pub(crate) enum FirmTrySendError {
	#[error("executor was configured without firm commitments")]
	NotSet,
	#[error("failed sending blocks to executor")]
	Channel {
	#[from]
	source: mpsc::error::TrySendError<ReconstructedBlock>,
	},
	}

┆Issue Number: ENG-1039