serve finalized checkpoint state on an api and use it to checkpoint start the client

[g11tech]

serve the checkpoint state in octect stream like how its done in beacon - use the similar api route to serve it
as well as use this to checkpoint start the client i.e. use this as an anchorstate to start the client

(so client should automatically sync to other peers using block by roots on the missing parents which is already the feature we have, but get it working if its not)