[BUG] Transform jobs do not resolve the backing indices of a data stream

[ketanv3]

Describe the bug
A transform job allows users to define index expressions as the source. These index expressions can resolve index names, index patterns, and index aliases to the list of corresponding concrete indices. The same resolution doesn't happen for data streams.

Suggestion:
Index expression resolution should resolve a data stream name to its corresponding backing indices. This will make the resolution behavior consistent with index aliases.

Additional context
This issue is similar to how index resolution didn't happen correctly for rollup jobs (#12). Since transform jobs were recently introduced, it wasn't fixed in the previous PR (#13).