Use SSZ by default for VC <--> BN requests

[rkapka]

Description

more and more endpoints now support SSZ, we support it beacon apis and builders will be supporting ssz soon ethereum/builder-specs#104

• for GET requests we should use this "Accept: application/octet-stream;q=0.95,application/json;q=0.9" kind of header. and beacon nodes should support this. (in prysm we are not respecting this. need to investigate)
• for POST requests, we need the cache. (cache should be per url, to support multiple beacon nodes. this is how lodestar implements it). We could have the cache expire after a while (like 2 weeks), but not strictly necessary. but the cache should reset after vc restart.

Describe the solution you'd like

- [x] add a flag for enabling ssz, add it as a new feature flag

• Create of list of endpoints that support vs not support SSZ for this feature

• --use-ssz and --use-json, for debugging purposes.

• /eth/v3/validator/blocks/{slot}

• /eth/v2/beacon/blocks

• /eth/v2/beacon/blinded_blocks

• create list of endpoints that can be done in ssz and which are not for the validator client

• have ssz fall back to json is not available & cache the response
  - add new condition either at the http client level or at the endpoint itself