added documentation for external-tables and fallback_schema (#7610)

Author: matthewshaver

website/docs/reference/resource-configs/teradata-configs.md

@@ -413,3 +413,94 @@ These steps collectively ensure that the valid_history strategy effectively mana
   )}}
   ```
   See [Collecting Statistics documentation](https://docs.teradata.com/r/76g1CuvvQlYBjb2WPIuk3g/RAyUdGfvREwbO9J0DMNpLw) for more information.
+
+## The external tables package
+
+The [dbt-external-tables](https://github.com/dbt-labs/dbt-external-tables) package is supported with the dbt-teradata adapter from v1.9.3 onwards. Under the hood, dbt-teradata uses the concept of foreign tables to create tables from external sources. More information can be found in the [Teradata documentation](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Table-Statements/CREATE-FOREIGN-TABLE). 
+
+You need to add the `dbt-external-tables` package as a dependency:
+
+```yaml
+packages:
+  - package: dbt-labs/dbt_external_tables
+    version: [">=0.9.0", "<1.0.0"]
+```
+
+You need to add the dispatch config for the project to pick the overridden macros from the dbt-teradata package:
+
+```yaml
+dispatch:
+  - macro_namespace: dbt_external_tables
+    search_order: ['dbt', 'dbt_external_tables']
+```
+
+To define `STOREDAS` and `ROWFORMAT` for external tables, one of the following options can be used:
+- You can use the standard dbt-external-tables config `file_format` and `row_format` respectively.
+- Or you can add it in the `USING` config as mentioned in the [Teradata documentation](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Table-Statements/CREATE-FOREIGN-TABLE/CREATE-FOREIGN-TABLE-Syntax-Elements/USING-Clause).
+
+For the external sources, which require authentication, you need to create an authentication object and pass it in `tbl_properties` as `EXTERNAL SECURITY` object. For more information on authentication objects, check out the [Teradata documentation](https://docs.teradata.com/r/Enterprise_IntelliFlex_VMware/SQL-Data-Definition-Language-Syntax-and-Examples/Authorization-Statements-for-External-Routines/CREATE-AUTHORIZATION-and-REPLACE-AUTHORIZATION).
+
+The following are examples of external sources configured for Teradata:
+
+```yaml
+version: 2
+sources:
+  - name: teradata_external
+    schema: "{{ target.schema }}"
+    loader: S3
+
+    tables:
+      - name: people_csv_partitioned
+        external: 
+          location: "/s3/s3.amazonaws.com/dbt-external-tables-testing/csv/"
+          file_format: "TEXTFILE"
+          row_format: '{"field_delimiter":",","record_delimiter":"\n","character_set":"LATIN"}'
+          using: |
+            PATHPATTERN  ('$var1/$section/$var3')
+          tbl_properties: |
+            MAP = TD_MAP1
+            ,EXTERNAL SECURITY  MyAuthObj
+          partitions:
+            - name: section
+              data_type: CHAR(1)
+        columns:
+          - name: id
+            data_type: int
+          - name: first_name
+            data_type: varchar(64)
+          - name: last_name
+            data_type: varchar(64)
+          - name: email
+            data_type: varchar(64)
+```
+
+```yaml
+version: 2
+sources:
+  - name: teradata_external
+    schema: "{{ target.schema }}"
+    loader: S3
+
+    tables:
+      - name: people_json_partitioned
+        external:
+          location: '/s3/s3.amazonaws.com/dbt-external-tables-testing/json/'
+          using: |
+            STOREDAS('TEXTFILE')
+            ROWFORMAT('{"record_delimiter":"\n", "character_set":"cs_value"}')
+            PATHPATTERN  ('$var1/$section/$var3')
+          tbl_properties: |
+            MAP = TD_MAP1
+            ,EXTERNAL SECURITY  MyAuthObj
+          partitions:
+            - name: section
+              data_type: CHAR(1)
+```
+
+### Fallback Schema
+The dbt-teradata adapter internally creates temporary tables to fetch the metadata of views for manifest and catalog creation. In cases you don't have permission to create tables on the schema you are working on, you can define a fallback_schema (to which you have the proper `create`/`drop` privileges) in the dbt_project.yml as a variable.
+
+```yaml
+vars:
+  fallback_schema: <schema-name>
+```