feat: make regex-uri able to use nginx variables for Plugin[Proxy-rewrite]

[V1endr3]

Description

Make regex-uri of proxy-rewrite able to use nginx variables.

Fixes #11224

Checklist

• [v] I have explained the need for this PR and the problem it solves
• [v] I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• [v] I have updated the documentation to reflect this change
• [v] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

Infos

1. Tests have been done in docker env, but no new test cases added.
2. This's my first PR for apisix & lua, if there is any bad smell of code, pls let me know. THX :D

[shreemaan-abhishek]

@V1endr3 please describe your proposal first in #11224

[V1endr3]

@V1endr3 please describe your proposal first in #11224

done, pls check it.

[shreemaan-abhishek]

I have marked this PR as a draft. We can start the review once we have a consensus from the community in #11224

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[github-actions]

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