feat(circuits): unify circuits compilation and zkey generation processes

[ctrlc03]

Description

This PR amends how we build circuits with custom parameters and how we generate the zkeys. Previously we used zkey-manager from the cli folder, now given we use circomkit for testing, we can also use it to build custom circuits and generate the zkeys. The goal is to simplify the process to users.

Should a user/developer wish to build the circuits and generate the zkeys for testing, all they would have to do now is to run the following from the root of the monorepo:

pnpm build:circuits-c / build:circuits-wasm
setup:zkeys

To generate custom circuits, this can be done by changing the circuits/circom/circuits.json file.

The documentation and ci workflows have been updated to reflect the changes.

Additional Notes

ps. I had to add big-integer as a dev dependency as it looks like it's needed by optimisedmt (and was shared by some dependency of zkey-manager before)

Confirmation

• I have read and understand MACI's contributor guidelines and code of conduct.
• I have read and understand MACI's GitHub processes.
• I have read and understand MACI's testing guide.

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[0xmad]

@ctrlc03 thanks, just some comments.