Add 'Static' type and improve type substitution codegen to accept it

[jsdw]

This PR:

• Adds a subxt::utils::Static<T> type If the type T impls Encode/Decode (ie static encoding and decoding), then the Static type itself implements EncodeAsType and DecodeAsType (which it'll do by statically encoding and decoding).
• Update type substitution to support concrete types and nested generic substitution (essentially to allow users to wrap their substitute types in subxt::utils::Static to make it possible to substitute in types which only impl Encode and Decode (albeit wrapped in this Static wrapper).
• Add a codegen CLI command to allow type substitution on the command line using the same logic (closes [feature request] Type substitution from CLI for codegen command #851).
• Move errors into a separate errors.rs module to be more consistent, and extend them with TypeSubstitutionErrors.

The overall reason for this is to help provide a path forwards for users who are currently using type substitution to pull in types which don't impl EncodeAsType and DecodeAsType (eg types from sp_runtime/sp_core etc), now that those traits are required to be implemented on generated types.

So a user can now do things like:

#[subxt::subxt(
    runtime_metadata_path = "/path/to/metadata.scale",
    substitute_type(
        type = "sp_runtime::multiaddress::MultiAddress<A, B>",
        with = "::subxt::utils::Static<::sp_runtime::multiaddress::MultiAddress<A, B>>"
    )
)]
pub mod node_runtime {}

For instance, to use sp_runtime's MultiAddress instead of our own Subxt implementation.

Note that, in general, it's better to rely on types which do impl EncodeAsType and DecodeAsType where at all possible (Subxt provides a utils::MultiAddress and utils::AccountId32 for instance which do impl these traits).

[lexnv]

The clippy CI failed step is unrelated to this PR, I've also encountered while #887 and found the commit helped resolve it.

[niklasad1]

can you file an issue this?

[niklasad1]

this is great