## Issue Addressed
None
## Proposed Changes
Adds support for downloading the deposit contract from a different location
by setting the environement variables `LIGHTHOUSE_DEPOSIT_CONTRACT_SPEC_URL`
and `LIGHTHOUSE_DEPOSIT_CONTRACT_TESTNET_URL`.
It also adds support to fetch the content from a local file:// URL.
This allows pre fetching to build in an environment without network access.
## Additional Info
Being able to build without network access is required to package the application for https://nixos.org/. But I imagine it might be useful for other distributions too.
## Issue Addressed
Closes#2274
## Proposed Changes
* Modify the `YamlConfig` to collect unknown fields into an `extra_fields` map, instead of failing hard.
* Log a debug message if there are extra fields returned to the VC from one of its BNs.
This restores Lighthouse's compatibility with Teku beacon nodes (and therefore Infura)
