Add brain API

[pablomendezroyo]

Add brain API for querying validators associated a tags/protocols.

API spec:

GET /api/v0/brain/validators?tag=lido&tag=lido&format=pubkey: get validators loaded in brain.

• tag (optional): string comma separated of "obol", "diva", "ssv", "rocketpool", "stakewise", "stakehouse", "solo", "stader", "lido". If not provided will be retrived from all tags
• format (required): one of "pubkey" | "index"

Response example

{
    "solo": ["validatorPubkey1", "validatorPubkey2"],
    "lido": ["validatorPubkey3", "validatorPubkey4"]
}