make boltdb-shipper use the generic indexshipper for managing index in object storage

[sandeepsukhani]

What this PR does / why we need it:
Make boltdb-shipper use the generic indexshipper for managing the index on object storage.
I have also moved index gateway client and its config to indexshipper since it is supposed to be used with tsdb as well.

Special notes for your reviewer:
Since this PR was getting too big, I will open a follow-up PR for the following changes:

1. Move code handling writes in pkg/storage/stores/shipper/index to pkg/storage/stores/shipper/index/writes package.
2. See if we can write index gateway benchmark test in a reliable way which is commented out to avoid circular dependency.
3. Add boltdb-shipper metrics to indexshipper and make it reusable for tsdb.
4. Rename pkg/storage/stores/shipper to pkg/storage/stores/boltdbshipper.

Checklist

• Tests updated

[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	-0.2%
+           chunkenc	0%
+              logql	0%
+               loki	0%

[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	-0.2%
+           chunkenc	0%
-              logql	-0.5%
+               loki	0%