Translate `HTTPHeaderValidationTests` to Swift testing

### Motivation:

We want to add tests that use parameterized testing to
`HTTPHeaderValidationTests`. In order to allow this we first need to
translate this file to use Swift Testing instead of XCTest.

### Modifications:

- Mechanical translation of `HTTPHeaderValidationTests` from XCTest to
Swift testing.

### Result:

We can add parameterized tests to HTTPHeaderValidationTests.

Add multi-version matrix support to Wasm Swift SDK workflow

### Motivation:

The WASM CI only ran against nightly-main, making it impossible to
distinguish toolchain regressions from code issues. Other repos in the
ecosystem using `swiftlang/github-workflows` already test against
`nightly-main`, `nightly-6.3`, and `6.3` by default.

### Modifications:

- Add `nightly_main_enabled`, `nightly_6_3_enabled`, and
`release_6_3_enabled` boolean inputs to `wasm_swift_sdk.yml` (all
defaulting to `true`, matching the coverage of
`swiftlang/github-workflows/swift_package_test.yml`) and update the
matrix generation to emit the appropriate entry per enabled version.
- Update `install_swift_sdk.sh` to support non-main branch snapshots by
deriving the API and download paths from `INSTALL_SWIFT_BRANCH` rather
than hardcoding `main`/`development`.
- Fail early with a clear message if the branch snapshot API returns no
results, rather than hitting an unbound variable error.

### Result:

WASM builds are now tested against multiple Swift versions, providing a
stable baseline to identify nightly-only regressions. Callers using the
workflow without arguments get the new default coverage automatically;
those who want to narrow the matrix can set specific version inputs to
`false`.

---------

Co-authored-by: Rick Newton-Rogers <rnro@apple.com>
Co-authored-by: Cory Benfield <lukasa@apple.com>

…#3567)

Split FoundationEssentials extensions from NIOFoundationCompat

### Motivation:

Resolves apple#3038 

### Modifications:

Split NIOFoundationCompat into two and only import FoundationEssentials
and NIOFoundationEssentialsCompat where appropriate.

### Result:

ByteBuffer/Codable extensions can be used without vendoring the code or
linking to Foundation.

Co-authored-by: Johannes Weiss <johannesweiss@apple.com>
Co-authored-by: Fabian Fett <fabianfett@apple.com>

apple#3567 (comment)

…ncRuntime is a new module that provides alternative implementations of MTELG and NIOThreadPool using Swift Concurrency. It compiles to wasm and runs in the browser.

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