Add Support for Explorer API

[alexcurtin]

Use-cases

the go-tfe library should support the new explorer API https://developer.hashicorp.com/terraform/cloud-docs/api-docs/explorer so users can run the same queries utilizing the go-tfe library

Attempted Solutions

I can write my own raw http calls to the endpoints, but this should be supported by go-tfe as its an official Terraform Cloud API.

Proposal

Add support for the endpoint, its response and query params/filters

[sebasslash]

Please see: #1018