remove bigtable_internal::InstanceChannelAffinityOption

Instead of using an Option internally, add a class member to hold the vector of instances.

Search the Bigtable codebase for `bigtable_internal::InstanceChannelAffinityOption` and inspect how its instance vector is created and consumed. Trace the surrounding class and its tests before changing the storage approach. Done means the internal option is no longer used and the equivalent instance data remains available through a class member, with the relevant Bigtable tests passing.

Scritto dal modello di indicizzazione a partire dal testo della issue.