[CT-2519] [CT-2442] [Bug] Foreign key constraint templating

[jtcohen6]

The most common syntax I see is:

• column level ("inline"): references {expression}
• model level ("out-of-line"): foreign key {column_list} references {expression}

Right now we're templating this out as foreign_key, instead of references.

Docs:

• https://www.postgresql.org/docs/current/ddl-constraints.html
• https://docs.snowflake.com/en/sql-reference/sql/create-table-constraint
• https://duckdb.org/docs/sql/statements/create_table.html
• etc

FWIW on BigQuery it does seem to be foreign_key and NOT references: https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#syntax_2

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If we were to take FK constraints more seriously, we're missing a pretty important ingredient, which is the ability to include & template ref inside the expression field — or providing more structure, i.e.

constraints:
  - type: foreign_key
    ref_table_name: ref('other_table_name')
    ref_column_names: ['id'] # could be multiple

Per #6754 (comment), we kicked that out of scope for v1.5, and we're unlikely to prioritize it while this remains a metadata-only (nonfunctional & unenforceable) feature on the majority of data platforms.