Move RateLimits method to a service

[WillAbides]

I missed the RateLimits method in #2937. It should be moved to a service struct.

Docs are at rate-limit/rate-limit#get-rate-limit-status-for-the-authenticated-user, so according to our existing pattern, we should move it to a new service called RateLimitService and name it Get

Client.RateLimits should be marked as deprecated and its body should be updated to return c.RateLimit.Get(ctx)

This may be a candidate for "good first issue" with the slight complication of creating a new service.

/cc #2936

[WillAbides]

For whoever picks this up, remember to add the new service here and to move the RateLimits tests from github_test.go to ratelimit_test.go.

There will be a small decline in test coverage because the newly deprecated Client method is no longer tested. The decreased test coverage will cause a failure status on your PR, but that can safely be ignored as long as the decreased coverage is only the Client method.

[nhAnik]

Hi @WillAbides, can I work on this?

[gmlewis]

Hi @WillAbides, can I work on this?

Thank you, @nhAnik ! It is yours.