Update appengine-testing open source build to 1.9.58
Update appengine-testing open source build to 1.9.58

The original impetus for this came from Kokoro build ID 57aa1042-65aa-46eb-9364-23e2bd17b2fd, where cron/TldFanoutActionTest.testSuccess_returnHttpResponse failed due to some strange nondeterminism in the fake task queue. This test isn't flaky in our build, so we assume it's been fixed in later implementations of appengine-testing.

This also adds a domain-registry-maven mirror for truth8 and clarifies the documentation for adding new libraries.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=173587282

google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus