Add macro for Service

[tdeleon]

Add a macro for an API service to allow for clearer code and reducing boilerplate. As part of this change, the Service typealias would be deprecated.
Example usage:

@RestAPI
enum MyAPI {
}

expands to

enum MyAPI: APIComponent {
}

The required baseURL could also be optionally provided through the macro:

@RestAPI("https://example.com/")
enum MyAPI {
}

expands to:

enum MyAPI: APIComponent {
   static let baseURL = URL(string: "https://example.com/)!
}

Note: Since macros require swift 5.9, an @available marker needs to be added to the macro declaration to avoid the entire package minimum swift version needing to be raised.

Documentation: #37