Make finality and block_to mutually exclusive in sync_chain_mmr endpoint

[sergerad]

I still think it would be good to make finality and block_to mutually exclusive - but I would leave this for a follow-up PR.

To illustrate the idea, in Rust, it would look something like this:

struct SyncChainMmrRequest {
    block_from: u32,
    block_to: UpperBlockBound,
}

// the name is just illustrative
enum UpperBlockBound {
    Block(u32),
    LastCommitted,
    LastProven,
}

Originally posted by @bobbinth in #1725 (comment)