feat: Set up user agent header for requests initiated from TF provider

[wjf3121]

So that we can have basic understanding of the provider usage through telemetry.

Some examples of how other providers do it:

https://github.com/hashicorp/terraform-provider-aws/blob/0faa8abd72bc2533defd52433cdc704d60d6e250/internal/conns/conns.go#L67-L79

https://github.com/hashicorp/terraform-provider-google/blob/6cd9e805ae13dbdf162b63b16bf4254165ac1ea9/google/transport/transport.go#L34-L36

https://github.com/confluentinc/terraform-provider-confluent/blob/d6298f5c7de36e1b5dd9d53b7d1cc638269e6476/internal/provider/provider.go#L443-L446