SSZ support for light client updates

[rkapka]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

Adds SSZ support to light client updates.

attested_header and finalized_header fields of updates are defined as a LightClientHeaderContainer, which itself is defined using oneof. This results in a non-exported interface type being generated and our SSZ generation library cannot handle this.

This PR resolves the above issue in the following way:

• We define an XXXInternal type for each XXX update type. It has the same fields as the original type, but the type of attested_header and finalized_header is replaced by bytes.
• We generate SSZ code for internal types.
• We manually write SSZ code for original types. This is done by converting the original type into the internal type and calling the appropriate method on the internal type. We convert attested_header and finalized_header into their "SSZ-bytes" representation by calling MarshalSSZ on these headers.

Which issues(s) does this PR fix?

Part of #12991

Acknowledgements

• I have read CONTRIBUTING.md.
• I have made an appropriate entry to CHANGELOG.md.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.