Community requested feature flags

[potuz]

Following up from internal Slack discussions. This PR implements several of the community requested feature flags.

• Adds a flag --optimistic-execution to allow the beacon node to run without an EL connected. Proposers must use an external builder. This results in approximately 1.4TB storage reduction in my locally tested node.
• Adds a flag --blobs-are-available to disable blob gossip, RPC requests and data availability validation. This results in 70% bandwidth reduction as well as extra 300Gb of storage.
• Adds a flag --allow-deep-reorgs which removes the 1-slot limit for the honest reorg feature. It should be safe to reorg longer branches as long as they haven't been voted heavily. This allows for larger MEV extraction if the previous proposers were slow or purposely broadcast their blocks late.

Other flags that are commonly requested are added but since they are not thoroughly tested yet I decided to leave them hidden. For the reviewer: if you think they are safe enough, we can change the visibility.