Declare in the document that ReadWriteSplitting Config is parsed by GroovyShell by default and adds corresponding unit tests

[linghengqian]

For #29000.

Changes proposed in this pull request:

• Declare in the document that ReadWriteSplitting Config is parsed by GroovyShell by default and adds corresponding unit tests.
• Also for Make ShardingSphere Proxy in GraalVM Native Image form available #21347 .

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