Support enforcing fields to string according to schema

[tuliren]

• When a field has string schema, force the field value to be a string.
• This is useful for array fields without item specification. In that case, the schema converter simply assumes that it is an array of strings.
• This is a required PR for 🐛 Jdbc sources: switch from "string" to "array" schema type for columns with JDBCType.ARRAY airbyte#8749.

[tuliren]

This is a redundant test case.