[CT-2218] [BigQuery] Implement constraint_support on BigQueryAdapter

[MichelleArk]

Core issue: dbt-labs/dbt-core#7067

Implement constraint_support on the BigQueryAdapter to satisfy:

constraint type	support
not_null	enforced
primary_key	supported
foreign_key	supported
unique	not supported
check	not supported

(from: dbt-labs/dbt-core#7067 (comment))

Additionally:

• Add the logic for templating out primary_key and foreign_key constraints within get_column_spec_ddl, per these docs (see primary key not enforced and references ... not enforced)
• Remove the constraints_check warning raised in get_columns_spec_ddl: https://github.com/dbt-labs/dbt-bigquery/blob/main/dbt/include/bigquery/macros/utils/get_columns_spec_ddl.sql#L13-L15

[amirbtb]

Hey folks ! Just wanted to inform that GCP announced in the 22/02/2023 Google Cloud release notes the beta of primary & foreign key table constraints.
Here are the limitations.

Thank you for this wonderful initiative around constraints (and of course all others) ! ♥️

[jtcohen6]

@amirbtb What timing! Thanks for flagging - I updated the description above.