Add custom derives for specific generated types

[ascjones]

Currently the user can only specify custom derives to be applied to all generated types. Sometimes it is required to use a specific generated type with certain requirements e.g. Ord, PartialOrd or serde::Serialize, serde::Deserialize, however it is not always possible/desirable to implement those on all generated types. For example when used in conjuction with substitute_type where the substituted type does not implement the required derives.

This PR adds the ability to specify derives for specific generated types, by adding the derive_for_type configuration to the subxt attribute:

#[subxt::subxt(
    runtime_metadata_path = "examples/polkadot_metadata.scale",
    derive_for_all_types = "Clone, PartialEq, Hash",
    derive_for_type(ty = "sp_runtime::DispatchError", derive = "::serde::Deserialize") // todo check this syntax
)]

Note also that generated_type_derives has been renamed to derive_for_all_types.

The final set of derives applied to a specified type will be the union of default_derives + user_derive_for_all_types + user_derive_for_type.

todo

• example
• unit tests
• test with cargo-contract
• docs

[jsdw]

This is great!