**What type of PR is this?**

> Feature

**What does this PR do? Why is it needed?**

Adds event emission for `execution_payload` as per
ethereum/beacon-APIs#588. Refactors
`ReceiveExecutionPayloadEnvelope` by decoupling EL validation from
signature verification+DA check to satisfy the event emission timing for
`execution_payload_available`.

**Which issues(s) does this PR fix?**

N/A

**Other notes for review**

- Renames the existing struct names clearly to indicate whether it is
for `execution_payload` or `execution_payload_available`.

**Testing**

Kurtosis config file:

```yaml
participants:
  - el_type: ethrex
    el_image: ethpandaops/ethrex:glamsterdam-devnet-4
    el_extra_params:
      - --http.api=eth,net,web3,admin
    cl_type: prysm
    cl_image: prysm-bn-custom-image:latest
    vc_image: prysm-vc-custom-image:latest
    supernode: true
    count: 3
    cl_extra_params:
      - --subscribe-all-subnets
      - --verbosity=debug
    vc_extra_params:
      - --verbosity=debug

network_params:
  fulu_fork_epoch: 0
  gloas_fork_epoch: 2
  seconds_per_slot: 6
  genesis_delay: 40

additional_services:
  - dora

global_log_level: debug
```

You can receive **both** events with this command:

```bash
➜  prysm git:(feat/execution-payload-event) curl -X 'GET' \
  'http://127.0.0.1:32813/eth/v1/events?topics=execution_payload&topics=execution_payload_available' \
  -H 'accept: text/event-stream'
```

Result:

```
event: execution_payload_available
data: {"slot":"64","block_root":"0x0ef955bd61d0e402bc71d6b97f223fa9377683dce64561069c408c5c2e0d548d"}

event: execution_payload
data: {"slot":"64","builder_index":"18446744073709551615","block_hash":"0xbf648d1f316e6d8284ed74c97c4942af1daf052f7517c5fca8b00836151e1d3d","block_root":"0x0ef955bd61d0e402bc71d6b97f223fa9377683dce64561069c408c5c2e0d548d","execution_optimistic":false}

event: execution_payload_available
data: {"slot":"65","block_root":"0xfec09760c4bd83adcff905cb95b732bfcef7c00f529f5e2471ff34db512fbdbe"}

event: execution_payload
data: {"slot":"65","builder_index":"18446744073709551615","block_hash":"0x6c327ec9e4de925b28eb5e127fbfae8d6082590199d35423ea95f8f0300c0423","block_root":"0xfec09760c4bd83adcff905cb95b732bfcef7c00f529f5e2471ff34db512fbdbe","execution_optimistic":false}
```

**Acknowledgements**

- [x] I have read
[CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
- [x] I have included a uniquely named [changelog fragment
file](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md#maintaining-changelogmd).
- [x] I have added a description with sufficient context for reviewers
to understand this PR.
- [x] I have tested that my changes work as expected and I added a
testing plan to the PR description (if applicable).