File change monitoring on s3_auth (#8905)

[moonchen]

1. Add a file change monitoring API to iocore.
2. Use it to add config change monitoring to s3_auth. This can be used to rotate s3_auth secrets.

(Previously I made PR #8905 against 9.2.x by mistake. It should have been against master branch. This PR is rebased on top of master. It also incorporates an autest change suggested by @bneradt.)

[bneradt]

Looks like the gold file doesn't quite match curl's output: https://ci.trafficserver.apache.org/job/Github_Builds/job/autest/1219/console

Probably good to replace lines that aren't strictly relevant to the test output with ``.

In case it's helpful, issues like this make me lean toward using Proxy Verifier in AuTests. It has a purpose built field verification mechanism rather than the generic gold comparison.

[moonchen]

My text editor trimmed trailing whitespace from the gold file. It looks like the other gold file to match ATS log output also failed. I'm investigating.

[moonchen]

Autest failure should be fixed with latest push. Thanks @bneradt

[zwoop]

Also, removing 9.2.x from this, we'll add this new API and plugin changes for 10.0.x.

[moonchen]

Couple of things to do before we can merge:

1. This needs to be discussed on the mailing list, as is the normal process for adding new plugin APIs. A short description of the new APIs, with the prototypes etc., and a justification.
2. These new APIs only work on Linux, which I feel reasonably strongly is not acceptable. I don't think we have any other APIs that are Linux only. So, we'll need to add kqueue support to this as well please :).

1. API description sent to mailing list.
2. kqueue implementation now added.

[github-actions[bot]]

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