agent: remove serviceRegiration type
agent: remove serviceRegiration type

Replace with the existing AddServiceRequest struct. These structs are
almost identical. Additionally, the only reason the serviceRegistration
struct existed was to recreate an AddServiceRequest.

By storing and re-using the AddServiceRequest we remove the need to
translate into one type and back to the original type.

We also remove the extra parameters to a function, because those values
are already available from the AddServiceRequest field.

Also a minor optimization to only call tokens.AgentToken() when
necessary. Previous it was being called every time, but the value was
being ignored if the AddServiceRequest had a token.

hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul