Backport of proxycfg: ensure that an irrecoverable error in proxycfg closes the xds session and triggers a replacement proxycfg watcher into release/1.15.x (#16529)
* backport of commit 88e56eac02
* backport of commit 4fc870086d
* backport of commit 0d57b2a70c
* backport of commit c627e5ac39
* backport of commit 48f93514de
* backport of commit 3b9fbcd800
* backport of commit a323375c30
* backport of commit 70b54d501f
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Co-authored-by: R.B. Boyer <rb@hashicorp.com>
Co-authored-by: R.B. Boyer <4903+rboyer@users.noreply.github.com>
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