[merge-to-stable-23-3] issue-2100, issue-757: implement fallback mode for filestore SS proxy; cloud/storage/core/libs/ss_proxy should be able to backup path descriptions; extract TSSProxyActor from cloud/filestore - this actor implements DescribeScheme, ModifyScheme and WaitSchemeTx requests + small several small fixes

[merge-to-stable-23-3] issue-2100, issue-757: implement fallback mode for filestore SS proxy; cloud/storage/core/libs/ss_proxy should be able to backup path descriptions; extract TSSProxyActor from cloud/filestore - this actor implements DescribeScheme, ModifyScheme and WaitSchemeTx requests + small several small fixes #673