Add gRPC example to mimic CheckJWT functionality

[kingscott]

📝 Checklist

• [x] All new/changed/fixed functionality is covered by tests (or N/A)
  • N/A for this example
• [x] I have added documentation for all new/changed functionality (or N/A)

🔧 Changes

• Provided an example of how to use a new middleware, in a gRPC context, that replicates the CheckJWT functionality

📚 References

• Relates to #181

🔬 Testing

• This example only provides a documented example of how this middleware can be used - the code is not runnable
• I opted to not provide tests for the middleware as they are quite lengthy and require a decent amount of boilerplate

[ewanharris]

Thanks for the PR @kingscott! I think given the warnings from snyk around some of the dependencies being pulled in via go-kit this might prove difficult to get in.

Do you think it would be feasible to provide a barebones gRPC example rather than one built ontop of go-kit?

[kingscott]

Hey @ewanharris - understandable. I'd need to dig in a bit more to how I'd do this; I've only really used go-kit for gRPC. Arguably, this PR could even be simplified to just show checking a JWT outside of a transport context? (Kind of like a helper method?)