[dbt-postgres] Microbatch strategy implementation: merge

[MichelleArk]

As part of dbt-labs/dbt-core#10624, dbt-postgres needs to implement a default microbatch strategy.

Although it is not write-optimized, we will use merge under-the-hood in dbt-postgres for now, as dbt-postgres does not yet support partitioning.

The base tests from dbt-adapters 1.6 should also be inherited and included as part of this work.