Fix to avoid using global actors as default values

[sidepelican]

Fixes #171 .

This PR includes two changes:

1. Fix for cases where any annotated existential types cannot find a default mock value.

Base

/// @mockable
protocol SomeProtocol { }

/// @mockable
protocol UseSomeProtocol {
    func foo() -> any SomeProtocol
}

Before

class UseSomeProtocolMock: UseSomeProtocol {
    ...
    func foo()  -> any SomeProtocol {
        fooCallCount += 1
        if let fooHandler = fooHandler {
            return fooHandler()
        }
        fatalError("fooHandler returns can't have a default value thus its handler must be set")
    }
}

After

class UseSomeProtocolMock: UseSomeProtocol {
    ...
    func foo()  -> any SomeProtocol {
        fooCallCount += 1
        if let fooHandler = fooHandler {
            return fooHandler()
        }
        return SomeProtocolMock() 
    }
}

2. The mock is may have a global actor doesn't provide a default value

Fix #171. Calling a function with a globalActor from a mock is very complicated, so it is easier to give up the default value.

[fummicc1]

Thank you so much! Implementation is nice. Just to confirm, I added some comments.

[fummicc1]

LGTM. Thank you!