Add support for custom docc "build" flags

[finestructure]

As discussed in the Documentation WG, in order to give authors some control over docc feature opt-in we want to support a way to let them specify custom flags that are passed on to swift package generate-documentation.

[finestructure]

We are currently supporting two different modes when building docs: via spm (macOS and Linux docs) and via xcodebuild (ios docs).

Via spm we run (essentially)

• swift package generate-documentation --target <target>

whereas xcodebuild does

• xcodebuild docbuild -scheme <target>
• xcodebuild docc process-archive transform-for-static-hosting

Which command needs the support for custom flags in case of ios doc builds? I'm hoping we can just have on entry that supports both doc build use cases.

[finestructure]

I'll assume it's the docc command for now, so a custom build command would go to

• swift package generate-documentation --target <target> $custom

or

• xcodebuild docc process-archive transform-for-static-hosting $custom

depending which platform the author has chosen.

[finestructure]

• merge SPIManifest changes Custom doc gen parameters SPIManifest#26
• merge Builder changes

[finestructure]

Ok, needs to be passed to docbuild via OTHER_DOCC_FLAGS:

% xcodebuild docbuild OTHER_DOCC_FLAGS='--diagnostic-level error --experimental-parse-doxygen-commands'

https://swift-open-source.slack.com/archives/C04PCMXMBD0/p1679675776691159?thread_ts=1679643396.399739&cid=C04PCMXMBD0

[finestructure]

This is live since Monday via builder 4.15.0