Add serde::serialize and serde::deserialize traits to signature types

[DOBEN]

Purpose

In dApp projects, custom smart contract types often need to be sent between backend and frontend where the request is JSON encoded. It is desired to derive these traits, especially for types from our standards such as PermitParam from the CIS3 standard. To be able to derive it for PermitParam, the AccountSignature type misses such feature. We need to implement these traits on the signature types in contracts-common first.

Changes

• Derive serde::Serialize and serde::Deserialize on signature types: AccountSignature, CredentialSignature, Signature, SignatureEcdsaSecp256k1, and SignatureEd25519.

[abizjak]

So it's quite annoying that a transitive dependency bumped the version to 1.73.

For simplicity we should just update rust-version in all our Cargo.toml files that specify it to 1.73.

And reflect it in the changelogs of the respective components.

It's annoying, but we are forced.