hosting and consuming checkpoint sync server API

[ch4r10t33r]

• Add /lean/states/finalized API endpoint to serve finalized checkpoint state as SSZ binary
• Add --checkpoint-sync-url CLI parameter to enable checkpoint sync from another zeam node
• Implement client-side checkpoint state download, verification, and anchor state initialization

Motivation
Closes #469

Checkpoint sync allows new zeam nodes to bootstrap from a trusted node's finalized state rather than syncing from genesis. This significantly reduces initial sync time for nodes joining the network.

Attached screenshot demonstrating the working of the endpoint

[anshalshukla]

I think --metrics-port basically runs the API server on that port and /metrics is an endpoint over API server. I think we should update flags to use --api-port, not sure if there is a standard that has to be followed here amongst all the clients and also this will lead to changes in quickstart as well.

[ch4r10t33r]

We should use std.http.Client here

Yes, I agree. We should address this and follow standard names. But, let me deal with it as a separate PR. I'll create an issue for this.

Created: #486

[anshalshukla]

LGTM!