Add data_source: internal_events for new API

Problem

Right now, any metrics, that we define or migrate as part of the new Gitlab Internal Events Tracking are not distinguishable from the old ones. E.g. the metrics migrated as part of #407858 (closed) are still on the redis_hll data_source.

Desired Outcome

We want to take one step more towards our proposed system by adding a new data_source: internal_events.

Proposed Solution:

1. Add data_source: internal_events as valid part of the metric schema.
2. Update the documentation around data_source to include internal_events
3. Make sure that data_source: internal_events still allows for aggregated metrics for unique counters (RedisHLL). We do not implement aggregation for total counts with this issue (see #411454 (comment 1391997025))