adds feature-gated serde traits to TransactionMetadata

[ebrightfield]

Another simple one. Adds feature-flagged serde traits on TransactionMetadata and FailedTransactionMetadata. This will be useful for use cases involving sending the transaction results over the wire.

The requisite serde traits are already added on struct fields coming from solana-sdk. (De-)serializing Signature to/from string.

I didn't add serde to the default features of the crate, up to you.

[ebrightfield]

Thank you for contributing, can you add it in changelog please?

My pleasure, done!

[kevinheavey]

Hey @ebrightfield just saw this, what is serde_with_str.rs for?

[Aursen]

The module to serialize the signature as a string

[kevinheavey]

But Signature already derives Serialize, no?

#[repr(transparent)]
#[derive(
    Serialize, Deserialize, Clone, Copy, Default, Eq, PartialEq, Ord, PartialOrd, Hash, AbiExample,
)]
pub struct Signature(GenericArray<u8, U64>);

[Aursen]

Yeah but serialized as an array