* 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.
The result will still pass gofmtcheck and won't trigger additional
changes if someone isn't using goimports, but it will avoid the
piecemeal imports changes we've been seeing.
