Lookup results by policy id

[KtorZ]

Describe your idea, in simple words.

Similar to #25, but regarding asset ids or policy id.

Why is it a good idea?

Similarly, it allows to make more out of the same indexer. Considering that #25 and this ticket would likely require indexes which may (a) make the database a lot larger and (b) slow-down the overall synchronization, it may be a good idea to only enable indexing on-demand (via configuration or compilation flags) or to allow disabling indexing if unused.

Are you willing to work on it yourself?

Yes.

[KtorZ]

Relates to #34, may be superseded by it.