use alloy beacon types

[ltitanb]

When we first implemented the PBS module, we had to implement all the types manually, but now alloy has rpc_types_beacon which we could use instead. The only thing missing currently is the tree_hash support, so we should open a PR for that directly in alloy

ref: alloy-rs/alloy#1024

[ltitanb]

we could also import types directly from https://github.com/sigp/lighthouse as they now use alloy as well