* Changes to importing configured callable functions

* Import a callable function only if it's present as a string in config

* Added missing return type annotations

* Better mechanism to import symbols defined in YAML config

* Docstrings, better logging

* cache_key_fn loadable from YAML, additional refactoring

* Cache key functions moved to workflow_utils.artifact.cache module
level
* Cache key functions renamed
* "current" and "tstamped" cache key functions removed from
workflow_utils.artifact.cache module
* Removed CacheKeyUtil class
* cache_key_fn now loadable from YAML configuration

* Refactoring of ArtifactCache classes

* Removed cache_key method from FsArtifactCache and replaced it with a
function as an argument to the constructor.
* Created utility class CacheKeyUtil for different cache key function
implementations.
* Removed FsVersionedArtifactCache class, replaced it with regular
FsArtifactCache with custom cache key functions.

* README.md update, docstrings updated, put method removed from FsArtifactSource

* README.md updated to reflect the refactored classes
* Missing docstrings updated for methods in source.py and cache.py
* put(artifact) method removed from FsArtifactSource
* MavenArtifactSource constructor now checks if base_uri is not None

* Recursive cache delete

* Enabled recursive cache artifact delete
* Added a relevant unit test

* Refactoring work done

* Since we have no plans on supporting underlying FS libraries other
than fsspec, abstract base classes ArtifactCache and ArtifactSource
have been removed
* Added URI validity check in cache and source constructors
* FsVersionedArtifactCache refactored to accept a callable argument
that provides the final component of the cache output path, instead of
automatically creating "current" and tstamped paths
* Switched to using fsspec.core.url_to_fs function to get a handle to
the underlying filesystem

* Better tests, pendulum removed, performance improvement

* tests/test_artifact.py improvements: hard-coded string for file
content put into a shared variable, better fixture naming, cleaner
comparison of source and cached folders/files
* Removed dependency on pendulum library, switched to simple datetime
* In versioned cache artifact, "current" copies from "tstamped" to
improve performance

* Another linter bug

* Fix linting issue with regular expression backslash

* Introducing versioned artifact cache

In this MR we introduce a new kind of cache - a versioned cache.

This cache stores copies of artifacts in two separate directories.
One directory is named by a timestamp at the time of caching, in
the YYYYMMDDHHmmss format, and the other directory is named "current".

So, where a normal cache would take an artifact like this:

rootfs://artifact_root_dir
|
+-- artifact.file

and place it in cache like this:

cachefs://cache_root_dir
|
+-- artifact.file

the versioned cache will produce a directory layout as follows:

cachefs://cache_root_dir
|
+-- 20241004121212
| |
| +-- artifact.file
|
+-- current
  |
  +-- artifact.file

This MR also changes the way the library uses fsspec API - instead of
opening a stream to write to caches, the library now uses fsspec's
`copy` method that can work on both files and directories.