use standard GenerateAccessTokenRequestString()

[WindzCUHK]

Description

• adapt: support proxyForPrincipal parameter in GenerateAccessTokenRequestString at athenzutils/ztsclient.go athenz#2457

Type of change

• Bug fix
• New feature
• Refactoring (no functional changes, no api changes)
• Non-code changes (update documentation, pipeline, etc.)

Flags

• Breaks backward compatibility
• Requires a documentation update
• Has untestable code

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Checklist

• Followed the guidelines in the CONTRIBUTING document
• Added prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip] in the PR title if necessary
• Tested and linted the code
• Commented the code
• Made corresponding changes to the documentation
• Passed all pipeline checking

Checklist for maintainer

• Use Squash and merge
• Double-confirm the merge message has prefix [skip ci]/[ci skip]/[no ci]/[skip actions]/[actions skip]
• Delete the branch after merge