support mTLS client certs when connecting to Grafana

[joemiller]

It would be useful if optional TLS client key/cert could be used for connecting to Grafana. This would allow management of Grafana instances behind a TLS proxy like nginx or ghostunnel.

I propose adding 3 new options to the provider configuration block:

• tls_key
• tls_cert
• ca_cert

All would be optional. But both tls_key and tls_cert must be used if either is used. ca_cert is optional. If it is specified the CA bundle will be used to validate the Grafana server's TLS cert. If unspecified, the default Go stdlib CA bundle would be used.

I'd be happy to work on a PR if the project might consider adding this functionality.

[gfdsa]

This also covers the custom CA use-case.

[joemiller]

implemented in #133
thank you