Provide TARGETPATH normalizing convenience interface

[lukpueh]

Description of issue or feature request:
#1008 removes ambiguity in interfaces that take TARGETPATH arguments, by only accepting paths and path patterns that use "/" as directory separator and don't start with "/", as recommended in the specification (see #1018 for even stricter vetting of these arguments).

To provide a friendly user interface we should add normalizing functionality that transforms paths as returned by os.listdir, os.walk, repository_tool.Repository.get_filepaths_in_directory, etc... into a format that is accepted by methods that take TARGETPATHs.

Normalizing includes

• strip targets directory prefix from absolute paths
• convert Windows-style dir separators "" to Unix-style separators "/"
• maybe perform os.path.normpath-like normalization, like collapsing redundant separators and up-level references

#957 has some discussions about least-suprise in TARGETPATH, e.g. what do do if an absolute path does not have the targets directory as prefix, or a relative path is passed, etc...

I envision an interface that may be used like this:

target_role.add_targets(normalize_target_paths(get_filepaths_in_directory(dir_path)))

or the with the latter two combined

target_role.add_targets(get_normalized_paths_in_directory(dir_path)))

[jku]

I'm not closing this directly, maybe there is something here... However, current implementation does try to consider target paths as URL fragments, that may or may not map to a local path

[lukpueh]

The add_target-like interface of the new Metadata API requires callers to be explicit about the target_file_path as it appears in the targets metadata, and the the local path, used to determine hashes and length of the target file to be added:

python-tuf/tuf/api/metadata.py

Lines 1545 to 1556 in 8ac7167

	def from_file(
	cls,
	target_file_path: str,
	local_path: str,
	hash_algorithms: Optional[List[str]] = None,
	) -> "TargetFile":
	"""Creates ``TargetFile`` object from a file.
	Args:
	target_file_path: URL path to a target file, relative to a base
	targets URL.
	local_path: Local path to target file content.

To keep the Metadata API minimal, I suggest to implement the requested convenience interface in a future repository tool. Since we don't have a roadmap for that yet, I'll close this issue and add a comment/reference in #1136.