Exposing TemporalityPreference constants

[dprotaso]

I had exposed TemporalityPreference strings that would map to TemporalitySelectorFuncs in the following PR: open-telemetry/opentelemetry-go#7346

There was some ambiguity where the preference constants should live
From: open-telemetry/opentelemetry-go#7346 (comment)

This issue is meant to resolve that ambiguity