This change adds metrics for evaluating editable context collectors and
the collectors themselves:

- `cursor excerpt`: small area around the cursor.
- `current-file`: the full file that contains the cursor.
- `edit-history`: excerpts around edit history edits.
- `edit-history-file`: full files referenced by the edit history.
- `git-log`: files related to the cursor file according to recent git
co-change history.
- `oracle-file`: full files referenced by the expected patches. This is
an oracle/evaluation-only source because it uses the target patch.

Individual collectors can be enabled by passing a comma-separated list
like this: `ep context --type=edit-history,current-file`. There's also a
preset, `--type=editable`, which will enable all collectors except
oracle.

This change also adds the
`.prompt_input.related_files[0].excerpts[0].context_source` field.

Editable context is only accessible via ep-cli for now.

Release Notes:

- N/A

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Co-authored-by: Eric Holk <eric@zed.dev>