JSON output makes it easy to process synchronization progress
information in downstream tools (e.g. the vscode extension).
This changes introduces a `sync.Event` interface type for progress events as
well as an `sync.EventNotifier` that lets the sync code pass along
progress events to calling code.
Example output in text mode (default, this uses the existing logger calls):
```text
2023/03/03 14:07:17 [INFO] Remote file sync location: /Repos/pieter.noordhuis@databricks.com/...
2023/03/03 14:07:18 [INFO] Initial Sync Complete
2023/03/03 14:07:22 [INFO] Action: PUT: foo
2023/03/03 14:07:23 [INFO] Uploaded foo
2023/03/03 14:07:23 [INFO] Complete
2023/03/03 14:07:25 [INFO] Action: DELETE: foo
2023/03/03 14:07:25 [INFO] Deleted foo
2023/03/03 14:07:25 [INFO] Complete
```
Example output in JSON mode:
```json
{"timestamp":"2023-03-03T14:08:15.459439+01:00","seq":0,"type":"start"}
{"timestamp":"2023-03-03T14:08:15.459461+01:00","seq":0,"type":"complete"}
{"timestamp":"2023-03-03T14:08:18.459821+01:00","seq":1,"type":"start","put":["foo"]}
{"timestamp":"2023-03-03T14:08:18.459867+01:00","seq":1,"type":"progress","action":"put","path":"foo","progress":0}
{"timestamp":"2023-03-03T14:08:19.418696+01:00","seq":1,"type":"progress","action":"put","path":"foo","progress":1}
{"timestamp":"2023-03-03T14:08:19.421397+01:00","seq":1,"type":"complete","put":["foo"]}
{"timestamp":"2023-03-03T14:08:22.459238+01:00","seq":2,"type":"start","delete":["foo"]}
{"timestamp":"2023-03-03T14:08:22.459268+01:00","seq":2,"type":"progress","action":"delete","path":"foo","progress":0}
{"timestamp":"2023-03-03T14:08:22.686413+01:00","seq":2,"type":"progress","action":"delete","path":"foo","progress":1}
{"timestamp":"2023-03-03T14:08:22.688989+01:00","seq":2,"type":"complete","delete":["foo"]}
```
---------
Co-authored-by: shreyas-goenka <88374338+shreyas-goenka@users.noreply.github.com>
Pieter Noordhuis