do not initialize tsdb index store when using index gateway client for queries

[sandeepsukhani]

What this PR does / why we need it:
When Loki is configured to run the index gateway client for doing tsdb queries, we also initialize tsdb index store instance just to satisfy the IndexStore interface here, which is just a fallback for keeping index gateway client and store backwards compatible.
This causes us to un-necessarily run an instance of indexshipper which would try to sync the files locally if query readiness is configured.

This PR changes the code to make the fallbackStore for NewIndexGatewayClientStore optional and keep it nil when initializing it for tsdb index store.

[owen-d]

After the build passes (bit of linting/etc) LGTM

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
-            storage	-0.5%
+           chunkenc	0%
+              logql	0%
-               loki	-0.1%