Make resource_connector_schema_config usable again!

[beevital]

Fix for https://github.com/fivetran/terraform-provider-fivetran/issues/263

To avoid such issues we introduced new fields in resource_connector_schema_config:

• Field schemas - map ["schema_name"] -> schema
• Field schemas_json - field accepts raw json schema settings. Example:

  resource "fivetran_connector_schema_config" "test_schema" {
      provider = fivetran-provider
      connector_id = "connector_id"
      schema_change_handling = "ALLOW_COLUMNS"
      schemas_json = <<EOT
      {
          "schema_0": {
              "enabled": true,
              "some_random_extra_field": "extra_value",
              "tables": {
                  "table_0": {
                      "some_random_extra_field": "extra_value",
                      "enabled": true
                  }
              }
          },
          "schema_2": {
              "enabled": true,
              "some_random_extra_field": "extra_value",
              "tables": {
                  "table_0": {
                      "some_random_extra_field": "extra_value",
                      "enabled": true,
                                                "columns": {
                                                    "column_1": {
                                                        "enabled":  false
                                                    }
                                                }
                  }
              }
          }
      }
      EOT
  }
  `

  To pass the value it's better to use file("path-to-schemas.json").

[jonathan-mothership]

On 1.1.17, looping over 10 connections and applying the following, we get "inconsistent results" and failed apply. Hoping this PR will help!

resource "fivetran_connector_schema_config" "schema" {
  for_each               = local.fivetran_connection_configs
  connector_id           = fivetran_connector.postgres_cons[each.key].id
  schema_change_handling = "ALLOW_ALL"
  schema {
    name    = "pglogical"
    enabled = "false"
  }
  schema {
    name    = "migrations"
    enabled = "false"
  }
}