* Vendor gravitational/trace/trail in api
Pulling in the trail package directly in api will allow the trace
module to shed the grpc-go dependency. This needs to land prior
to https://github.com/gravitational/trace/pull/112 being included
in a new version of trace.
There should be no noticable change in the api depdency tree since
it already depends on grpc-go. Some additional items from the
trace/internal package were also vendored within trail as needed.
Additionally, some of the public api of trail that was not being
consumed has been made private.
* fix: appease linters