Use BLS aggregate signatures for validator registry

[Marchhill]

This PR makes it much faster and cheaper to submit validator registrations.

To register n consecutive validators with consecutive indices would previously take n submissions of 142 bytes. With this PR we can register all n with a single 142 byte submission. This has several advantages:

• Much faster to submit
• Won't cause rate limiting due to spamming an endpoint with many submissions
• Cheaper gas costs
• Less signatures need to be verified by validators / keypers

[konradkonrad]

This is a pretty clever and elegant way to make this change, without needing too much other changes. Looks good!

It would be great to exchange some test vectors for the aggregate signature format before moving forward.

[konradkonrad]

Looks good - we have it implemented like this and works in testing.

[konradkonrad]

@Marchhill looks like I don't have Approval rights in this repos. But from our side this is good to go!

[dapplion]

@konradkonrad please throw an approve on related PRs to you even if you don't have perms