Add a FileDigest type to differentiate file APIs from directory APIs

[stuhood]

Problem

Two codepaths in our @rule API use a Digest for a file rather than a directory, and this can cause confusion, since using a Digest of the wrong type will trigger a failure to lookup a digest in storage. More generally, our Python API has so far been more typesafe than our Rust API with regard to Digests: fairly consistently using them to represent Directorys.

Solution

Introduce FileDigest, and use it in the two relevant locations:

1. DownloadFile takes the FileDigest for the file being downloaded.
2. single_file_digests_to_bytes takes FileDigests returned in workunits.artifacts (see Fix retrieval of bytes from artifacts in StreamingWorkunitHandler #10698).

Result

Improved type safety and clarity. Fixes #10898.

[ci skip-build-wheels]

[stuhood]

A lot of the churn in this PR is due to the addition of these type checks, because it required threading the Types instance in a few more places.

I expect that in the medium term we should consider moving all of the fs.py types to Rust using cpython's class generation mechanism, as it's likely both safer (better error messages) and more efficient than the externs::project_* boilerplate:

pants/src/rust/engine/src/externs/interface.rs

Lines 463 to 488 in 4129e06

	py_class!(class PyExecutionStrategyOptions |py| {
	data options: ExecutionStrategyOptions;
	def __new__(
	_cls,
	local_parallelism: u64,
	remote_parallelism: u64,
	cleanup_local_dirs: bool,
	speculation_delay: f64,
	speculation_strategy: String,
	use_local_cache: bool,
	local_enable_nailgun: bool
	) -> CPyResult<Self> {
	Self::create_instance(py,
	ExecutionStrategyOptions {
	local_parallelism: local_parallelism as usize,
	remote_parallelism: remote_parallelism as usize,
	cleanup_local_dirs,
	speculation_delay: Duration::from_millis((speculation_delay * 1000.0).round() as u64),
	speculation_strategy,
	use_local_cache,
	local_enable_nailgun,
	}
	)
	}
	});

See http://dgrunwald.github.io/rust-cpython/doc/cpython/macro.py_class.html for more info.

[coveralls]

Coverage remained the same at 0.0% when pulling 60c4acb on stuhood:stuhood/add-and-use-file-digest into 4129e06 on pantsbuild:master.