feat: add support for more CL keystore formats

[ManuelBilbao]

Currently, Commit-Boost only support Lighthouse-like keystore, with a file structure similar to:

├── keys
│   ├── <PUBLIC_KEY_1>
│   │   └── voting-keystore.json
│   └── <PUBLIC_KEY_2>
│       └── voting-keystore.json
|── secrets
│   ├── <PUBLIC_KEY_1>
│   └── <PUBLIC_KEY_2>

This PR adds support for many other formats, like Prysm, Teku and Lodestar.
Solves a part of #17

[ltitanb]

Overall looks good! Can we also add some tests using testdata generated by https://github.com/protolambda/eth2-val-tools