Add new host function to check the number of keys on the account

[frol]

This is useful when you want to check if the contract account is locked (no keys are there)

I have initially considered some API that would allow to read all the keys in the contract context, but it seems quite trouble-some from the encoding point of view. A simple number or even a bool ("are there any keys on the account?") would already help to solve the security challenge that is is front of https://github.com/galactic3/name_hub project, which needs to ensure that nobody else can control the account but the trusted contract deployed to it.

There is a more high-level problem, that I described in #5740

cc @matklad @evgenykuzyakov @mikedotexe @amgando @nearmax @bowenwang1996

[matklad]

Adding fn acces_key_count() -> Result<u64> seems good to me. If the need arises, we can layer "read all keys" on top of it later (see fn promise_results_count(&mut self) -> Result<u64> and fn promise_result(&mut self, result_idx: u64, register_id: u64) -> Result<u64>).

[frol]

I feel it can return u64 (I don't see how it can fail), but other than that it is exactly what I am looking for. We might want to have two counts: the number of any access keys, and the number of full access keys.

[matklad]

(I don't see how it can fail)

It can run out of gas

EDIT: I am being confusing here -- internal impl would return a Result, as all host functions do. For the contracts, the signature would be (import "env" "access_key_count" (func (result i64))) and it'll panic in case of an error.

[frol]

I was under impression that running out of gas won't get control back to Wasm code anyway 🤔

Looking at signer_account_pk() -> PublicKey I assumed there should not be the Result on the API layer while some things might indeed go wrong on the ABI layer which will cause panic.

[bowenwang1996]

Duplicate of near/NEPs#79