Support certificate authentication for RESTful sensors

Currently, RESTful sensors only support authentication methods of basic or digest. It would be great if it could support certificate client authentication.

My use case for this is that I have some other self-hosted services which have APIs and I’d like to pull that data into home assistant, but I generally like to have as many of my services behind client authentication as possible. So I can’t use these with home assistant unless I disable client authentication for them.

My thoughts on implementation would be to add a new type like client_certificate, and add a new configuration variable (something like certificate_file) which would point to a pkcs12 file (containing the client cert and private key). You’d also need a way to specify the password for that pkcs12 package, maybe that could just use the existing password variable, or add a new one. Alternatively, there could be two options, one that points to the certificate file and one that points to the key file (similar to ssl_certificate/ssl_key in the http integration).