* use automtls for v5 secrets/auth plugins
* add automtls env guard
* start backend without metadata mode
* use PluginClientConfig for backend's NewPluginClient param
refactor
* - fix pluginutil test
- do not expect plugin to be unloaded in UT
- fix pluginutil tests --need new env var
- use require in UT
- fix lazy load test
* add changelog
* prioritize automtls; improve comments
* user multierror; refactor pluginSet for v4 unit test
* add test cases for v4 and v5 plugin versions
* remove unnecessary call to AutoMTLSSupported
* update comment on pluginSets
* use runconfig directly in sdk newpluginclient
* use automtls without metadatamode for v5 backend plugin registration
* use multierror for plugin runconfig calls
* remove some unnecessary code
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