✨ feat: support to set client in proxy mw

[li-jin-gou]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.
Explain the details for making this change. What existing problem does the pull request solve?

Fixes #2116

• In some scenarios the user need set up some complex client config via custom client
• The case of a global client only gives the user no way to set up special configurations for only one middleware

so this pr allow to use custom client and distinguish global client and custom client in proxy mw and compatible with previous logic.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• update of the docs repository

Commit formatting:

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[ReneWerner87]

please don´t forget

• the update of the docs repository

[li-jin-gou]

please don´t forget

• the update of the docs repository

Thanks for reminding. 😭
already modified doc.