spiffe identifier support

**Is your feature request related to a problem? Please describe.**
Managing certificates/identities for all your log shippers / log processors is difficult.

**Describe the solution you'd like**
Integrate with spire. https://spiffe.io/

Have a plugin for spire so that it fetches/refreshes certificates and trust bundles from the spire-agent via socket.

Also allow validation of spiffe identifiers by pattern so that mutual trust can be established between log shipper
and log processor

And finally, a way to mutate the spiffe identifier accepted into the log lines would be nice too, so you know for sure where the log line came from.

**Describe alternatives you've considered**
Proxies used between log shipper and log processors