Extract shared indexed-list decoders for beacon REST handlers

[syjn99]

What type of PR is this?

Bug fix & Refactor

What does this PR do? Why is it needed?

As we've added SSZ support for request body, there are some duplicated logics for decoding SSZ/JSON (decode*{SSZ,JSON}). This PR adds a common decoding functions both for SSZ and JSON based on the callback function passed.

This PR also fixes a bug that might be panicked when there's null entry in JSON body. (An unit test added: null element keeps failure indices aligned) Also, POST /eth/v2/beacon/pool/attestations now doesn't compact the valid attestations; instead, it defines the fixed length of slice and puts valid one via index.

Which issue(s) does this PR fix?

N/A

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).