feat: implement DynSSZ support

[filoozom]

Implements support for DynSSZ thanks to attestantio/go-eth2-client#123.

This is very useful for non-standard networks, like devnets, testnets or specifically Gnosis Chain and Chiado in our case. This way, all the configuration variables are fetched automatically from the nodes and used to decode data and calculate roots.

I'm not a Go developer, so this is probably not the right way to do things. However, it does work and demonstrate its usefulness. A few issues for sure:

• Only fetching the data from one node instead of using them all
• Fetching the spec on every calculation instead of caching it
• Error management

I'm hoping that someone can help me finish this in an idiomatic way!