Support @unchecked Sendable

[crane-hiromu]

Thank you for creating this library 🤝

Is your feature request related to a problem? Please describe.

Coucnrrency compliance is required in Xcode 15.3 or later.
Compliance with Sendable will result in a warning in the generated Spy.

• example（on Xcode15.3）

Describe the solution you'd like

You can solve this by adding @unchecked Sendable to the generated Spy.

• example code

class XxxxCaseProtocolSpy: XxxxCaseProtocol, @unchecked Sendable {

or

extension XxxxCaseProtocolSpy: @unchecked Sendable {}

Describe alternatives you've considered

It would be nice if implementers could optionally choose whether to unchecked or not.

• example code

@Spyable(behindPreprocessorFlag: "DEBUG", uncheckedSendable: true)

I was going to make my own PR, but it seemed difficult to modify it with my knowledge, sorry...

I hope you'll address this issue.
Thank you 👍