…ng to be persisted (#36619)

* Add TestDesanitizeRemovesAllFakeSettings to catch future omissions

Walks every string field in the config after a Sanitize+desanitize
round-trip and fails if any still holds FakeSetting. This catches the
case where a field is added to Sanitize without a corresponding
desanitize entry.

* Fix ElasticsearchSettings.ClientKey being incorrectly masked as a secret

ClientKey is a file path, not a secret value. Masking it caused the
asterisk string to be persisted to the database on config writes, which
broke TLS client auth on restart.

* Fix desanitize missing entries for fields added in 504fb96

504fb96 masked five fields in Sanitize without adding the
corresponding desanitize entries, meaning a config save through the
API would permanently overwrite those fields with FakeSetting:

- FileSettings.ExportAmazonS3SecretAccessKey
- ServiceSettings.GoogleDeveloperKey
- ServiceSettings.GiphySdkKey
- CacheSettings.RedisPassword
- AutoTranslationSettings.LibreTranslate.APIKey

* fixup! Add TestDesanitizeRemovesAllFakeSettings to catch future omissions

* fixup! Fix desanitize missing entries for fields added in 504fb96

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Co-authored-by: Mattermost Build <build@mattermost.com>