GitHubAppCredentials on remote side to use SlaveToMasterCallable

[jglick]

More verbose but probably safer version of #329. Untested.

The idea here is that the agent never sees the private key in plaintext, nor does it have the opportunity to create a spoofed API endpoint or whatever. It just gets an opaque, encrypted cookie, and any time it wants to look up the app token, it must ask the controller. Compared to #329 this is stateless.

Rather than Secret, which can be constructed in other ways, I would rather use CryptoConfidentialKey. Unfortunately that has IV-related methods which are still not available for use from plugins. There does not seem to any easy-to-use and safe way of constructing an encrypted cookie in Jenkins, much less one from structured data like this.

[jglick]

java.lang.AssertionError: expected:<1> but was:<0>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:633)
	at org.jenkinsci.plugins.github_branch_source.ApiRateLimitCheckerTest.NormalizeThrottleWithBurnedBuffer(ApiRateLimitCheckerTest.java:361)

seems to be a flake.

[bitwiseman]

This looks great to me. Do you mind if I add the caching back into this PR?

[jglick]

Do you mind if I add the caching back into this PR?

Sure, feel free to use this as a starting point for something more polished.