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/Cargo.toml

@@ -38,3 +38,4 @@ zeroize = { version = "1.4.2", features = ["zeroize_derive"] }
 lighthouse_metrics = { path = "../../common/lighthouse_metrics" }
 lazy_static = "1.4.0"
 ethers-core = { git = "https://github.com/gakonst/ethers-rs", rev = "02ad93a1cfb7b62eb051c77c61dc4c0218428e4a" }
+builder_client = { path = "../builder_client" }