[Feature Request]Adding compatibility support for KingbaseES

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Hi asyncpg maintainers,

I am writing to propose adding support for KingbaseES, a widely used database in China that is highly compatible with the PostgreSQL wire protocol. I plan to contribute and maintain this adaptation over the next 2 months.

To ensure the performance and stability of asyncpg remain unaffected, I will follow these principles:
1.Non-Invasive Implementation: I will not modify the Cython-based protocol parser (asyncpg/protocol/). The changes will be isolated to Authentication tweaks and Type Introspection SQL adjustments (handling minor pg_catalog differences).
2.Strict Testing: I will run the full asyncpg test suite against KingbaseES and add dedicated integration tests for any variant-specific edge cases.
3.Maintenance Commitment: I fully understand the burden of supporting variants. I commit to taking long-term ownership of Kingbase-related issues to keep the driver up-to-date.

I have already verified basic connections locally. Would the community be open to accepting a Pull Request for this? Also, do you prefer handling such variants via automatic detection (server version) or explicit connection flags?
Thank you for your incredible work on this high-performance driver!

Start by reviewing asyncpg/protocol/ and the existing Authentication and Type Introspection SQL paths to identify the PostgreSQL assumptions relevant to KingbaseES. Compare those paths with the reported pg_catalog differences and decide whether server-version detection or an explicit connection flag is appropriate. Done means basic connections work, variant-specific integration tests pass, and the full asyncpg test suite remains stable against KingbaseES.

Written by the indexing model from the issue text.