feat: Return block authentication data with GetBlockHeaderByNum

[igamigo]

Closes #256

[bobbinth]

Looks good! Thank you! I left a couple of comments with questions inline.

Also, let's:

• Update changelog.
• Update docs in RPC and Store readme files.

[bobbinth]

Shouldn't proof be an optional field? (based on the flag in the request).

Also, would returning just the Merkle proof be enough? Or should we include forest value as well?

Lastly, let's add comments to the fields here. I'd like to improve inline documentation (to be similar to SyncStateResponse documentation)

[igamigo]

Added the comments, and the forest which I had not taken into account.

For the optional field based on the flag, I wasn't sure so I was going to address it later, but I ended up adding the flag in the request to include authentication data. I think defaulting to omitting it is the better option out of the two (versus including it by default) since it incurs in less overhead (if your estimation of 2-4KB is correct, it probably dominates the response size). I don't have much of a strong opinion either though, so I'm open to changing it. I am also not sure if the bool in the request should be optional to enable the scenario of not even including it in the request.

[bobbinth]

Looks good! Thank you! I left a couple more comments inline.

[bobbinth]

Looks good! Thank you! I left some comments inline - all related to updating the use of forest in docs/comments.

[bobbinth]

All looks good! Thank you!