1st amendment.
"Congress shall make no law abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble"
Last I checked the mayor of New York is not Congress and assembling in a restaurant is peaceful.
To some degree forcing the 6-ft limit and mask in an abridgement of the right to assemble in a restaurant or a beach or any outdoor venue.
If you make the argument that they are doing this on behalf of people's health then apply it to everybody and everywhere.