configstore: add utility library functions for configstore

this change adds a set of library functions to facilitate
accessing(retrieving) configuration items.

Specifically, added template functions for accessing configuration
items for following types:

OptionalBool, OptionalInt32, OptionalUInt32, OptionalInt64,
OptionalUInt64, OptionalString

Design doc: go/design-confighal
Bug: 34724435
Test: build, run

Change-Id: Icca56d0d9e086b9d94c9b1168df041bf7d418698