experimental "local fuzzing" support (#405)

This PR adds an experimental "local" mode for the agent, starting with `libfuzzer`.  For tasks that poll a queue, in local mode, they just monitor a directory for new files.

Supported commands: 
* libfuzzer-fuzz (models the `libfuzzer-fuzz` task)
* libfuzzer-coverage (models the `libfuzzer-coverage` task)
* libfuzzer-crash-report (models the `libfuzzer-crash-report` task)
* libfuzzer (models the `libfuzzer basic` job template, running libfuzzer-fuzz and libfuzzer-crash-report tasks concurrently, where any files that show up in `crashes_dir` are automatically turned into reports, and optionally runs the coverage task which runs the coverage data exporter for each file that shows up in `inputs_dir`).

Under the hood, there are a handful of changes required to the rest of the system to enable this feature.
1. `SyncedDir` URLs are now optional.  In local mode, these no longer make sense.   (We've discussed moving management of `SyncedDirs` to the Supervisor.  This is tangential to that effort.)
2. `InputPoller` uses a `tempdir` rather than abusing `task_id` for temporary directory naming.
3. Moved the `agent` to only use a single tokio runtime, rather than one for each of the subcommands.
4. Sets the default log level to `info`.  (RUST_LOG can still be used as is).

Note, this removes the `onefuzz-agent debug` commands for the tasks that are now exposed via `onefuzz-agent local`, as these provide a more featureful version of the debug tasks.

src/api-service/__app__/onefuzzlib/tasks/scheduler.py

@@ -154,7 +154,7 @@ def build_work_unit(task: Task) -> Optional[Tuple[BucketConfig, WorkUnit]]:
         job_id=task_config.job_id,
         task_id=task_config.task_id,
         task_type=task_config.task_type,
-        config=task_config.json(),
+        config=task_config.json(exclude_none=True, exclude_unset=True),
     )
 
     bucket_config = BucketConfig(