* plugin/auth: enable multiplexing
- the plugin will be multiplexed when run as an external plugin
by vault versions that support secrets/auth plugin multiplexing (> 1.12)
- we continue to set the TLSProviderFunc to maintain backwards
compatibility with vault versions that don't support AutoMTLS (< 1.12)
* enable multiplexing for secrets engines
* add changelog
* revert call to ServeMultiplex for pki and transit
* Revert "revert call to ServeMultiplex for pki and transit"
This reverts commit 755be28d14b4c4c4d884d3cf4d2ec003dda579b9.
