There is no plug-in specifically for Mid/Side processing in that way, but it can be done quite easily.
There is a Mid/Side encoder/decoder plug-in here: https://forum.audacityteam.org/t/converter-for-sony-microphone-ecm-ms957/21847/13
To equalize the mid / side channels separately, split the stereo track into two mono tracks (click on the track name and select “Split to Mono”) then use the Equalization effect (Effect menu). To recombine the mono tracks back into a stereo track, click on the name of the upper track and select “Make Stereo Track”.
If you just want to adjust the “width” of a stereo track, there are much easier ways to do it.