proxy-rewrite plugin doesn't support “var” filter

[Sun Qi]

Hi Dev team，
    I need this plugin to support "var" conditional filtering to add a
request header for some special requests, but I found that this plugin does
not support it, but another plugin “response-rewrite” supports "var"
conditional filtering. My question is whether it is technically possible,
and the second is whether it is necessary.  I plan to add this feature.