feat: Add swift wasm compilation support to swift-distributed-tracing

[scottmarchant]

Summary

This PR adds support for compiling swift-distributed-tracing to wasm using the SwiftWasm sdk.

This PR is part of a larger effort by a company called PassiveLogic to enable broad support for SwiftWasm.

Details

There are three changes required to enable wasm compilation for swift-distributed-tracing.

• Removed unused import Dispatch imports. SwiftWasm doesn't currently include GCD. Luckily, all usage of GCD was unused.
• Added some missing wasi-libc and pthread imports
• Add shim to allow wrapped access to CLOCK_REALTIME struct from WASILibc. This is a critical piece of distributed tracing to acquire timestamps that needed some manual shim code to map to the matching API in WASI.

Testing done

• Cleaned up swiftformat lint on modified lines of change.
• Verified unit tests still pass with these changes
• Verified swift build completes without errors
• Verified no new compiler warnings are added with these changes
• Verified swift build --swift-sdk wasm32-unknown-wasi completes without errors
• Verified swift build --swift-sdk wasm32-unknown-wasip1-threads completes without errors
• Verified a third-party executable can build this library as part of a larger wasm executable using the command swift package --swift-sdk wasm32-unknown-wasip1-threads js --use-cdn

Impact Risk

Realistically, this change should not cause any impact or risk. Outside of wasm targets, the changes in this PR are additive.

[scottmarchant]

Note, there is a lot of existing swiftformat lint in this repository, including the existing indentation on many of these import blocks. I fixed the lint on the lines of code I'm changing in this PR. But I didn't fix it for everything else, to avoid making the diff far too large.