Taskcluster-taskgraph

Fixed
- Allow JSON-e in schema identifiers

Added
- `run-task` script sets a `TASK_WORKDIR` env pointing to the task specific working directory
- Add `defer` and `enforce_single_match` arguments in `util.schema.resolve_keyed_by`

Fixed
- Misspelling in job transforms causing utility file to be unnecessarily imported
- Added `if-dependencies` to `job_description_schema` allowing it to be used with job transforms

Changed
- Refactored logic for importing sibling modules into new `util.python_path.import_sibling_modules`

Perf
- Skip schema validation when `taskgraph.fast` is set
- Improved performance of `schema.optionally_keyed_by` function

Added
- Support for a `toolchain-env` key which gets added to dependent tasks' environment
- Support for `generic-worker` toolchain tasks
- Ability to execute commands using powershell in `run_task` transforms
- Implicitly run toolchain scripts ending in `.ps1` with powershell

Fixed
- Renaming `toolchain-artifact` now rebuilds toolchain tasks

Fixed
- Git checkout cleaning regression
- Add a mechanism to bypass `--require-hashes` when installing pip dependencies in `run-task`
- Decode gzipped artifacts when using `task-id=` or `project=` in the `--parameters` flag
- Stop defaulting to a dummy `TASKCLUSTER_ROOT_URL` and instead raise error
- Some node dependency upgrades in the `index-task` image

Added
- Ability to pass headers alongside requests in the `fetch-content` script

Fixed
- An encoding error when using out of tree images
- Windows permissions errors in `run-task` script cleanup

Added
- Add ability to run new types of verifications (e.g doc verifications)
- Allow custom `onExitStatus` values for generic-worker based tasks across all platforms

Changed
- Decision docker images updated to Ubuntu 20.04 and Mercurial 5.3.1
- Pass parameters into verifications functions

Fixed
- Don't assume Taskcluster is enabled in generic-worker run-task based tasks
- Exception during local generation when generic-worker tasks exist