Apply rename_all casing to RPC parameter names including a compat shim

[Eligioo]

Extend #[service(rename_all = …)] and #[proxy(rename_all = …)] to rename RPC parameter names on the JSON wire including an automatic alias in order to retain backwards compatibility.

This PR supersedes #30 as it's non-breaking.

The three commits:

• Rename method parameters to the configured casing — params now follow the same rename_all convention as method names (e.g. first_operand → firstOperand).
• Generate a serde alias for backward compatibility — instead of renaming the Rust identifier, the macro keeps idents snake_case and emits #[serde(rename = "<new>", alias = "<old>")] per field. Servers therefore will accept both the new and the original name during the migration window. This also fixes a compile-time panic for rename_all = "kebab-case" (a hyphen is not a valid Rust identifier).
• Warning at runtime when a legacy parameter name is used — the dispatcher emits a single log::warn! per request when a request still uses a pre-rename parameter name. Is a no-op when no params were renamed.

Examples

Old style still being accepted when rename_all is set to camelCase because of the alias for compatibility. Note: in this case a log::warn is outputted on the server side since at least one parameter isn't in camel case.

{
	"jsonrpc": "2.0",
	"method": "addNumbers",
	"params": {
		"first_operand": 2,
		"second_operand": 3
	},
	"id": 1
}

New style is also supported when rename_all is set to camelCase

{
	"jsonrpc": "2.0",
	"method": "addNumbers",
	"params": {
		"firstOperand": 2,
		"secondOperand": 3
	},
	"id": 1
}