add flag for prysm to use SSZ encoding for sending REST calls ( at least where we can)

[james-prysm]

🚀 Feature Request

Description

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

we should have a flag turned on so that we can test our requests and responses in SSZ.

Describe the solution you'd like

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

• have ssz fall back to json is not available

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

• add a flag for enabling ssz, add it as a new feature flag

• add new condition either at the http client level or at the endpoint itself,

• add some helper functions to make it easier + take a looka t how builder api does it.

Describe alternatives you've considered

[rose2221]

i would like to work on this issue!

[james-prysm]

closing in favor of 15317