[core] Only one LambdaRuntime.run() can be called at a time (fix #507)

[sebsto]

This is a proposal to fix issue #507

changes

• LambdaRuntime.init() uses a Mutex<Bool> to make sure only one instance is created
• LambdaRuntime.init() can now throw an error in case an instance already exists (I did not use fatalError() to make it easier to test)
• All convenience init() methods catch possible errors instead of re-throwing it to a void breaking the user-facing API
• Renamed existing LambdaRuntimeError to LambdaRuntimeClientError
• Introduced a new type LambdaRuntimeError to represent the double initialization error

[sebsto]

Compiler crashes on nightly build has been reported here
swiftlang/swift#80020

[sebsto]

CI is now green, except API Breakage, which is expected.

[fabianfett]

Great spot! Thanks for tackling this one.

[sebsto]

@fabianfett
I changed

• Mutex for Atomic
• Test is now on run() and not on init()
• LamabdaRuntimeError returned and is now public + one additional error case
• unit test is adjusted and pass

[sebsto]

Now that managing cancellation is working (thank you @adam b2811a5)

This PR is ready to review before merged.
API Breakage is expected as we are adding an error case.

implemented

[adam-fowler]

Looks good, only a couple of minor issues