feat(limit-req): support variables for rate/burst and multi-rules configuration

[niemne]

Description

This PR enhances the limit-req plugin by introducing support for dynamic variables and multiple rule sets, allowing for more flexible and granular rate limiting strategies.

Key Changes:

1. Dynamic Variables: The rate and burst parameters now accept string variables (e.g. ${http_rate}), enabling dynamic threshold adjustments based on request attributes.

2. Multi-Rule Support: Introduced a new rules configuration array. This allows defining multiple rate limiting rules (containing rate, burst, and key) within a single plugin instance.

3. Logic Refactoring: Core logic has been refactored into apisix/plugins/limit-req/init.lua to handle the new rule resolution and processing.

Which issue(s) this PR fixes:

relate issue: #13179

Checklist

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