Limit number of `SmtProof`s in `get_account` API

[drahnr]

When returning SmtProofs we should be more restrictive than the MAX_ENTRIES threshold, since they're both big in size and need more CPU cycles to obtain.

#1428 (comment)

[bobbinth]

I think the limit should actually be placed on the number of keys that can be requested - so that we can reject things at request time.

Another interesting aspect, somewhat related to this: we can try to get more clever and in some cases returning all entries rather than SMT proofs. For example, if a storage map has 20 entries - returning all 20 key-value pairs will almost always be cheaper than returning 10 SMT proofs. But that's probably for a different issue.

[bobbinth]

In #1517, we've implemented a store-level check that limits the number of returned SMT proofs. We should also implement a similar check in the RPC (before the request gets forwarded to the store), and ensure that in both cases we use the same constant.