Extract `ic_agent::identity` module into a standalone crate

[wiyota]

The ic-auth-client crate currently depends on ic-agent. However, it actually only uses the identity module within it. Despite this limited usage, version updates to the entire ic-agent crate require breaking changes and updates to ic-auth-client due to compatibility issues.

If you could publish the identity module as an independent crate, similar to ic_principal, it would reduce the maintenance overhead for ic-auth-client and the burden on its users. Could you please consider this request?