Remove builder redundancy (#3294)

## Issue Addressed

This PR is a subset of the changes in #3134. Unstable will still not function correctly with the new builder spec once this is merged, #3134 should be used on testnets

## Proposed Changes

- Removes redundancy in "builders" (servers implementing the builder spec)
- Renames `payload-builder` flag to `builder`
- Moves from old builder RPC API to new HTTP API, but does not implement the validator registration API (implemented in https://github.com/sigp/lighthouse/pull/3194)



Co-authored-by: sean <seananderson33@gmail.com>
Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/execution_layer/src/metrics.rs

@@ -3,6 +3,7 @@ pub use lighthouse_metrics::*;
 pub const HIT: &str = "hit";
 pub const MISS: &str = "miss";
 pub const GET_PAYLOAD: &str = "get_payload";
+pub const GET_BLINDED_PAYLOAD: &str = "get_blinded_payload";
 pub const NEW_PAYLOAD: &str = "new_payload";
 pub const FORKCHOICE_UPDATED: &str = "forkchoice_updated";
 pub const GET_TERMINAL_POW_BLOCK_HASH: &str = "get_terminal_pow_block_hash";