Add limit to how many SSH keys a user can have in their account

As per #4764, a user can transfer up to 100 SSH keys to an instance but technically has no limit on how many keys can be added to their account. This means that there exists a scenario in which a user has 100+ keys and expects all of those keys to get transmitted to the instance but only the first 100 get transmitted. The current behavior is to silently succeed whereas we eventually either want to have the same limit of keys per instance / keys per account or have some other means of ensuring this scenario doesn't happen. It may be that the work in #4919 removes the particular edge case here whereby just setting some top level limit would be sufficient.

See this comment for more details: https://github.com/oxidecomputer/omicron/pull/4764#discussion_r1467884930