docs(adr-116): modular crypto

[melekes]

Refs #2424

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[zmanian]

Many of the advantages of alternate cryptography require more invasive changes to the code base and even the header structure.

For instance to get advantages from BLS-12-381, you also need to be able use aggregated public keys in the commit certificates.

There are wins like a more EVM verification friendly comet that uses keccack and ethsign for signatures that can maybe be done without a hard fork.

[github-actions]

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[sergio-mena]

Although still very preliminary, I support this approach

[sergio-mena]

I'd rather use the KeyType (a string) that every crypto package must define, and which is supposed to be unique. Examples here, here, or here.