Allow specifying backup execution node URLs
**Is your feature request related to a problem? Please describe.**
Currently the only way to provide execution node URLs is via `--web3-urls` flag. This flag as far as I understand drives all provided ELs so they can sync, but uses only one at a time in a round-robin fashion based on availability.

This approach works but has several flaws:
- It does not allow for having lower-priority EL nodes that might be in a different location(latency issues).
- It does not switch URLs unless the currently used one fails, which means it does not go back to the first one.

**Describe the solution you'd like**
I would propose having `--fallback-web3-urls` or `--backup-web3-urls` which are used only in case all of `--web3-urls` fail. This would allow for operators to run a limited number of backup synced EL nodes that are normally not used, unless the primary one the BN is attached to fails, and would be not used when it recovers.

Benefits of this approach:
- Ability to fall back to an EL that normally should not be used, which is useful in a few configurations:
    - In small setups where main local EL is preferred to a remote backup EL in a separate geographical location.
    - In big setups where you have `A` beacon nodes(ex.: 10 for exmple) with `B` fallback ELs(ex.: 2) which limits costs.

**Describe alternatives you've considered**
I have considered using Nginx reverse proxy with upstream that contains [`backup`](https://nginx.org/en/docs/http/ngx_http_upstream_module.html#example) servers:
```nginx
upstream backend {
    server localhost:8551; # Local default EL

    server fallback-el-01.over.vpn:8551 backup;
    server fallback-el-02.over.vpn:8551 backup;
}
```
Benefits:
- Nginx would automatically return to using local EL if it becomes available again.
- More fine-grained control thanks to settings like `weight` or `max_fails`.

Issues:
- Nginx cannot detect RPC API errors returned from a faulty EL node, only timeouts, while a BN could.
- Lack of good BN metrics indicating which EL node is being used and how it performs.

**Additional context**
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