Provider: Add mTLS support

[magodo]

This PR adds mTLS support. It also introduce a breaking change to re-structure the provider configuration:

• Introduce a new client single nested attribute
• Moves the cookie_enabled and tls_insecure_skip_verify from top-level under client
• Add a new list nested attribute client.certificates, which contains:
  • certificate
  • certificate_file
  • key
  • key_file
• Add two new attributes client.root_ca_certificates and client.root_ca_certificate_files

Issues Fixed

• Fix #7
• Fix #59