Make SwiftSDKMetadataV4.sdkRootPath optional

[marcprux]

Makes the "sdkRootPath" property of swift-sdk.json optional.

Motivation:

The Android SDK bundle (swiftlang/swift#80788) does not include the Android NDK's sysroot in the bundle itself, but instead relies on it being installed locally. The install location will vary, and the user will be able to configure it with a command (modulo #8584) like:

swift sdk configure --sdk-root-path ~/Library/Android/sdk/ndk/27.0.12077973/toolchains/llvm/prebuilt/darwin-x86_64/sysroot/ swift-6.2-RELEASE-android-0.1 aarch64-unknown-linux-android28

However, since the SwiftSDKMetadataV4.sdkRootPath property is declared as non-optional, some value must be included in the swift-sdk.json file.

@MaxDesiatov at swiftlang/swift#80788 (comment) mentions:

As for making it optional, I don't quite remember the exact issue that caused it to become non-optional. After all, making it optional is technically not a breaking change, so potentially could be considered if necessary.

Modifications:

Change SwiftSDKMetadataV4.sdkRootPath from String to String?

Result:

The swift-sdk.json can now contain destinations that do not specify a sdkRootPath property.

[MaxDesiatov]

Would you mind adding tests that verify this change? Thanks!

[rauhul]

Changes like this 100% need to have accompanying documentation, otherwise anyone reading the SE is going to be extremely confused.