[CT-2625] Support model contracts + constraints with nested fields in core

[MichelleArk]

dbt-labs/dbt-bigquery#738 implemented model contract + constraint support for models with nested fields in BigQuery, but much of the implementation is not specific to any adapter. The adapter-specific piece relates to formatting the nested column type (so this bit).

BigQuery's support for nested columns is very thorough - supporting many levels of nesting (up to 100) and constraints on nested columns. Nested columns are not supported with the same maturity across various platforms and so it feels to early to be implement, test, and maintain this spec across all adapters. We could eventually revisit this though - and remove the need for the BigQuery adapter to implement overrides of get_empty_schema_sql and get_select_subquery macros.