Move to async implementations of `UserToolchain.init` and `SwiftSDK.hostSwiftSDK` #84

Job	Run time
Test / macOS (Xcode 26.0 - sequoia - ARM64)	3m 41s
Test / iOS (Build Only, Xcode 26.0 - sequoia - ARM64)	4m 6s
Test / Linux (nightly-main - bookworm)	10m 44s
Test / Linux (nightly-main - jammy)	9m 55s
Test / Linux (nightly-main - noble)	10m 22s
Test / Linux (nightly-main - amazonlinux2)	10m 45s
Test / Linux (nightly-main - rhel-ubi9)	10m 56s
Test / Windows (nightly-main - windows-2022)	29m 17s
Test / Swift SDK for Android Build (${{ matrix.swift_version }} - ${{ matrix.os_version }} - NDK ${{ matrix.ndk_version }})	-1s
Soundness / Broken symlinks check	5s
Soundness / YAML lint check	8s
Soundness / Documentation check	44s
Test / Embedded Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	-1s
Soundness / Python lint check	10s
Test / Static Linux Swift SDK Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	-1s
Soundness / API breakage check	0s
Soundness / Shell check	0s
Test / Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	-1s
Soundness / Unacceptable language check	-1s
Soundness / Format check	-1s
Soundness / License headers check	0s
	1h 30m 47s

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