[FEAT] Add new config parameter to set batch-size for registration validators on relays

[blevkivskyi-everstake]

To improve performance when dealing with a high number of keys, we propose adding a new configuration parameter to the mux config like lighthouse validator.

A error when sending a large number of keys to a relay will be fixed::

HTTP error response: 413 / Failed to buffer the request body: length limit exceeded

[ank-everstake]

Example flow:

5000 keys on VC(validator client)
1 beacon node (BN)
3 relays
    1000 allowed validator keys to use #x relay
    1000 allowed validator keys to use #y relay
    3000 allowed validator keys to use #z relay

batch_size for relays
    100  #x relay
    200  #y relay
    1000 #z relay


Beacon node   requests to register 5000 keys
commit-boost  receives 5000 keys
commit-boost  makes requests to relays in batches
    10 requests*100 keys to #x relay
    5   requests*200 keys to #y relay
    3 requests*1000 keys to #z relay

Possibly we will also ask to rate-limit misbehaving VC(such as Lighthouse) that sends requests for registrations for each slot rather than each epoch.
Maybe under separate flag.