feat: As a user, I want to use Nginx variables in proxy-rewrite.regex_uri

### Description

Currently, `proxy-rewrite` supports NGINX variable resolution in `uri` config parameter, **but not in `regex_uri`**. See [APISIX Docs | Plugins - proxy-rewrite](https://apisix.apache.org/docs/apisix/plugins/proxy-rewrite/#attributes) for reference.

Examples:

- 🟢 Nginx variable resolution in `uri`:

```yaml
proxy-rewrite:
uri: /api/$arg_name
```

- 🟢 Pattern variable resolution in `regex_uri`:

```yaml
proxy-rewrite:
regex_uri:
- ^/api/(.*)/users/(.*)$
- /v2/$1/external-users/$2
```

- 🔴 Nginx variable resolution in `regex_uri`:

```yaml
proxy-rewrite:
regex_uri:
- ^/api/(.*)/users/(.*)$
- /v2/$1/external-users/$arg_name
```

This is **not** allowed and will throw an error.

## Question

Since `regex_uri` already supports variables resolution for regex patterns (e.g. patterns matched by `(.*)/(.*)` can be referenced with `$1` and `$2`), I was wondering: why does `uri` allow Nginx variables resolution, but `regex_uri` does not? Is this a design choice? Are there any contraindications in implementing this feature?

Start with the proxy-rewrite plugin attributes in the linked APISIX documentation and trace the existing uri and regex_uri handling in the repository. Compare Nginx variable resolution with regex capture references such as $1 and $2; done means a supported regex_uri replacement can resolve an Nginx variable without breaking existing capture behavior, with coverage for the provided examples.

Written by the indexing model from the issue text.