The parameter dbType in PageUtil is fixed and cannot be extended

[dsomehan]

• I have searched the issues of this repository and believe that this is not a duplicate.

Ⅰ. Issue Description

I want to extend a third-party database using spi，but the parameter dbType in PageUtil is fixed and cannot be extended

Ⅱ. Describe what happened

If there is an exception, please attach the exception trace:

Just paste your stack trace here!

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. xxx
2. xxx
3. xxx

Minimal yet complete reproducer code (or URL to code):

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• JDK version(e.g. java -version):
• Seata client/server version:
• Database version:
• OS(e.g. uname -a):
• Others:

[slievrly]

I searched for pageSql and countSql method references and found that it was only referenced in the test class. Why are these two methods used?