Add AsyncSemaphore data structure

[Supereg]

Problem

In some cases, it is very useful to ensure exclusive access in an asynchronous context (e.g., ensure that an async method within an actor is only entered by a single called at a time).
SpeziBluetooth introduced a AsyncSemaphore data structure within the latest release, see here.

Solution

This data structure would be great to reuse across the Spezi framework ecosystem (e.g., there is currently also a need in StanfordSpezi/SpeziLLM#45).
This would require to move out the data structure from SpeziBluetooth, add documentation and dedicated unit testing for within SpeziFoundation.

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines

[philippzagar]

AsyncSemaphore has been added to SpeziFoundation in: #7

[PSchmiedmayer]

Thank you @philippzagar!