Add constraint support for G2 and Fq2 for bn254

[CPerezz]

Thanks for this PR! Quick question: I see that the PR adds variables for Fq and G1, but not Fq2 or G2, or for the corresponding pairing; @CPerezz would you be interested in making a followup PR that adds these components? It should be straightforward to adapt the BLS12-377 code for this. Thanks!

Originally posted by @Pratyush in #737 (comment)