Generate Datastore indexes automatically

[dmdashenkov]

See README.MD for the indexes description.

[MykytaPimonovTD]

It is important to keep in mind that we can generate only some of the required indexes automatically:

1. Indexes for InboxMessage, when Datastore is used to store transport messages.
2. Indexes for aggregates read by the framework itself, since these indexes are identical for all aggregates.

What we cannot predict are the indexes required for client entity queries. For example, if an application reads a projection using multiple columns, Datastore may require a composite index. Because these queries are defined as a server call, the framework cannot generate the corresponding indexes automatically at startup.

If the framework generates only its own indexes while users maintain a separate index file for application queries, the following issue may arise:

If user cleans up what they believe are obsolete indexes from their file, they may inadvertently delete indexes required by the framework, causing it to stop functioning correctly.

I suggest considering a different approach:

Allow users to specify an index configuration file when creating a DatastoreStorageFactory. The framework could then validate queries against this configuration during initialization (or query registration) and fail fast if a required application index is missing. At the same time, the framework would continue to generate and manage the indexes required for its own internal queries.