Add build tooling so we can work in mozilla-central.

[mhammond]

Most of the files here are ignored when in application-services, but when copied into mozilla-central they enable "mach build" to build our megazord and other tooling.

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Branch builds: add [firefox-android: branch-name] to the PR title.

[mhammond]

@glandium I'd love your eyes on this if you have a sec. I'll get a more formal review from the build peers as this gets ready to actually land in m-c, but the earlier I get feedback the better.

[glandium]

Nothing particularly seems to be wrong in this patch. You'll probably have to discover the horror show that is using nss in rust tests, where you'll probably have to look at what e.g. neqo crates do.