Allow customizing namespace separator (e.g. `.` instead of `_`)

[YakupAltay]

Hi,

Currently, the #[rpc(namespace = "foo")] attribute generates method names like foo_bar. However, some APIs (e.g. Celestia Node) expect namespaced methods in the format foo.bar.

It would be helpful to support a namespace_separator option to customize this behavior, e.g.:

#[rpc(namespace = "foo", namespace_separator = ".")]

Would this be possible in a future release?

Thanks!

[YakupAltay]

Resolved at #1544