Add ability to query for child spans based on their parent spans

### Problem Statement

I want to query for all of the spans that are child spans of a root span (or trace or transaction) that has tags or measurements. Right now, you can only reference a root-span/transaction via the `transaction` searchable field. This is useful, but i'd like to be able to tag a transaction with a value and query for spans related to that value.

### Solution Brainstorm

filter: `transaction.tag[tag-name]:tag-value` or `transaction_tags.tag_name:tag-value`. Then `span.description would be all of the child spans that meet that criteria.

### Product Area

Explore

Start in Explore by reviewing how the existing transaction searchable field resolves spans and how span.description results are queried. Compare that behavior with the proposed transaction.tag[tag-name]:tag-value and transaction_tags.tag_name:tag-value filters. Done means users can find child spans through tags or measurements on their root span, trace, or transaction.

Written by the indexing model from the issue text.