Introduce new R2DBC Dialect for GaussDB

[liubao68]

GaussDB is a trustworthy data foundation for mission-critical workloads, where high availability, performance, security, scalability, and intelligence are non-negotiable and OpenGauss is the community version for GaussDB.

This issue is to implement a new R2DBC Dialect for both GaussDB and OpenGauss and provide test cases based on community OpenGauss.

[schauder]

We don't have the capacity to support more databases.
Please consider hosting the dialect in a separate repository.

We hope to eventually provide support for testing new dialects.

See: #2048