This adds the directory reloader implementation of the CrlProvider. This
will periodically reload CRL files in a directory per [gRFC
A69](https://github.com/grpc/proposal/pull/382)
Included in this is the following:
* A public API to create the `DirectoryReloaderCrlProvider`
* A basic directory interface in gprpp and platform specific impls for
getting the list of files in a directory (unfortunately prior C++17,
there is no std::filesystem, so we have to have platform specific impls)
* The implementation of `DirectoryReloaderCrlProvider` takes an
event_engine and a directory interface. This allows us to test using the
fuzzing event engine for time mocking, and to implement a test directory
interface so we avoid having to make temporary directories and files in
the tests. This is notably not in `include`, and the
`CreateDirectoryReloaderCrlProvider` is the only way to construct one
from the public API, so we don't expose the event engine and directory
details to the user.
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Co-authored-by: gtcooke94 <gtcooke94@users.noreply.github.com>
Gregory Cooke