[CT-2219] [Spark] Implement constraint_support on SparkAdapter

[MichelleArk]

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

Implement constraint_support on the SparkAdapter to satisfy:

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

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

Additionally, remove the not_null-related warning raised in spark__alter_column_set_constraints : https://github.com/dbt-labs/dbt-spark/blob/main/dbt/include/spark/macros/adapters.sql#L202-L204