feat: add header to installing extension with the build mode so we know where the installation is coming from

[khendrikse]

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Summary

This adds a user agent header to our installation of auto-installable extensions.

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[eduardoboucas]

So the user agent will be 'buildbot' | 'cli' | 'require'? For future-proofness, do we want to be a bit more descriptive and include the fact that we're calling from Netlify Build, potentially including the version?

[khendrikse]

hmm yeah, it's not always netlify build though, could be the call to netlify config they do (we still have the double call thing)

[ndhoule]

I'd suggest something like:

User-Agent: Netlify Build (mode:${extensionInstallationSource}) / ${buildVersion}

If there's any other arbitrary metadata that might be useful to us later in analysis, we can include it semicolon-delimited in the parenthetical.

[eduardoboucas]

I think it would be nice to unify this with how we're doing it in build and avoid relying on a CJS feature:

build/packages/build/src/utils/json.ts

Lines 14 to 18 in 750c4b3

	export const importJsonFile = async function (filePath: string): Promise<PackageJson> {
	const fileContents = await readFile(filePath, 'utf-8')
	return JSON.parse(fileContents)
	}

[eduardoboucas]

Is this so that you can require the package name and not a relative path to the JSON file?