Remote query: add option to disable default/legacy indexing per schema file

All types that are not annotated are currently fully indexed for backward compat with first version of remote query (which did not have protobuf annotations to control indexing).

This behaviour is very inefficient and confusing for users that do not intend to use indexing (non-indexed query works anyway).

Given the history of this behaviour we cannot remove it until next major version. Until then we can offer the choice of disabling it per each schema file via a boolean protobuf file-level option named 'enable_legacy_indexing' 'indexed_by_default', which when absent is considered to default to true. Setting it to false will disable indexing of types that do not have indexing annotations.

Whenever an entry is indexed using default indexing a warning will be logged in order to motivate people to switch to using proper annotations.