Add RuntimeAPI for Rate Limit Checks

[camfairchild]

Is your feature request related to a problem? Please describe.

Currently, submitting weights, registering networks, and staking/unstaking (among others) are rate-limited. It would be useful to be able to check this rate limit via an API without having to send off a transaction and receive an error.

Describe the solution you'd like

Add a Runtime API that returns whether a specific key (if applicable) is within the rate limit for a next extrinsic.

Describe alternatives you've considered

• The alternative is to pull the rate limit value and the current usage from storage and duplicate the check for the rate limit on the client-side. But this would be difficult if any changes are made on the chain-side later-on and would involve a level of coupling.

Additional context

No response