[SPARK-12966][SQL] ArrayType(DecimalType) support in Postgres JDBC

Fixes error `org.postgresql.util.PSQLException: Unable to find server array type for provided name decimal(38,18)`.

* Passes scale metadata to JDBC dialect for usage in type conversions.
* Removes unused length/scale/precision parameters from `createArrayOf` parameter `typeName` (for writing).
* Adds configurable precision and scale to Postgres `DecimalType` (for reading).
* Adds a new kind of test that verifies the schema written by `DataFrame.write.jdbc`.

Author: Brandon Bradley <bradleytastic@gmail.com>

Closes #10928 from blbradley/spark-12966.