[cryptography/bn254] Add Support for bn254

[patrick-ogrady]

In #379, we created a proof-of-concept for producing bn254 aggregate signatures compatible with EigenLayer's smart contracts.

We should find the right place to support this stylized aggregation (uses aggregation trick and opinionated map-to-curve).

Maybe something like an ecosystem folder that contains implementations of Schemes (or other traits). Another approach could be much more scalable to manage if we expect ecosystems (or members of them) to host their own stuff.