Support Backfills for a subset of Tasks

### Description

We want the ability to support backfilling only a subset of tasks. Ideally this would be done via a task regex parameter or something similar

### Use case/motivation

We have DAGs that write multiple datasets out, and we frequently have cases where we wish to backfill on a specific dataset (therefore a specific task or task(s))

Another example is sensors for historical data, where the external sensors may depend on data that is no longer there (i.e. task sensors after the metadata db has had historical data deleted for performance etc.)

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

No files, tests, or entry points are named. Start by locating Airflow's backfill command and task-selection handling, then define how selecting tasks by regex should work for dataset backfills and sensors, including tests that demonstrate the intended subset behavior.

Written by the indexing model from the issue text.