Simplify checkpoint sync configuration

[nalepae]

🚀 Feature Request

Description

Currently, when syncing from a checkpoint, Prysm needs 2 flags:

• --checkpoint-sync-url, and
• --genesis-beacon-api-url

Very often (always?) these 2 flags are set to the exact same value.

Describe the solution you'd like

@kasey idea:

• If both flags are set, then use these two values respectively (no change).
• If --checkpoint-sync-url is set and --genesis-beacon-api-url is not, then use the value provided to --checkpoint-sync-url for --genesis-beacon-api-url.

[nalepae]

Actually it is already the case. ==> Need to modify the doc accordingly