Implement Workload API client

[azdagron]

This issue is to implement a primitive wrapper around the Workload API as outlined in client.go and options.go

For the most part, the implementation should more or less be a wrapper around the Workload API gRPC client, converting to/from protobufs and convenience types as necessary. All of the methods on the client are synchronous and straightforward.

Much of the dialing/setup code can be pulled from existing go-spiffe implementations.

The convenience methods in convenience.go should also be implemented as part of this issue. They are all small wrappers that:

1. create a new client
2. call the appropriate method
3. shut down the client.

This issue depends on #55, #56, #57, #58, #62, and #67.

[azdagron]

Fixed.