Define trait TransactionEnvelope

[emhane]

Moves part of implementation for TxEnvelope into new trait TransactionEnvelope.

[mattsse]

what's the main motiviation for this?

we haven't seen a usecase where we'd need to deal with various txenvelope types, although imo this trait seems reasonable.
but this currently clashes with txenvelope's native functions which we could then remove

[mattsse]

this bakes in the signed type, which I think is fine because I think this will not be used by reth.

but the function name could be problematic, could we introduce as_signed_* instead?

ah I guess this isn't necessary:

alloy/crates/consensus/src/transaction/envelope.rs

Line 282 in f390d44

pub const fn as_eip2930(&self) -> Option<&Signed<TxEip2930>> {

[emhane]

relying on this trait instead of hardcoded TxEnvelope type, allows for rollups to define their own TxEnvelope type to use in block bodies. it has same benefits as generalising the hardcoded transaction type in reth to T: alloy_consensus::Transaction.