Add vcfa_certificate resource and data source

[adambarreiro]

Add vcfa_certificate resource and data source.

This new config snippet must be added to the testing JSON:

    "certificates": [
      {
        "path": "../test-resources/cert.pem",
        "privateKeyPath": "../test-resources/key.pem",
        "password": "test"
      },
      {
        "path": "../test-resources/cert2.pem",
        "privateKeyPath": "../test-resources/key2.pem",
        "password": "test"
      }
    ],

[Didainius]

Test and manual testing passed.