[Security - Revocation] Crl Directory Watcher Implementation (grpc#34749

)

This adds the directory reloader implementation of the CrlProvider. This
will periodically reload CRL files in a directory per [gRFC
A69](grpc/proposal#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.

---------

Co-authored-by: gtcooke94 <gtcooke94@users.noreply.github.com>

include/grpc/grpc_crl_provider.h

@@ -28,7 +28,6 @@
 #include "absl/strings/string_view.h"
 
 #include <grpc/grpc_security.h>
-#include <grpc/support/sync.h>
 
 namespace grpc_core {
 namespace experimental {
@@ -68,6 +67,17 @@ class CrlProvider {
 absl::StatusOr<std::shared_ptr<CrlProvider>> CreateStaticCrlProvider(
     absl::Span<const std::string> crls);
 
+// Creates a CRL Provider that periodically and asynchronously reloads a
+// directory. The refresh_duration minimum is 60 seconds. The
+// reload_error_callback provides a way for the user to specifically log or
+// otherwise notify of errors during reloading. Since reloading is asynchronous
+// and not on the main codepath, the grpc process will continue to run through
+// reloading errors, so this mechanism is an important way to provide signals to
+// your monitoring and alerting setup.
+absl::StatusOr<std::shared_ptr<CrlProvider>> CreateDirectoryReloaderCrlProvider(
+    absl::string_view directory, std::chrono::seconds refresh_duration,
+    std::function<void(absl::Status)> reload_error_callback);
+
 }  // namespace experimental
 }  // namespace grpc_core
 
@@ -81,5 +91,4 @@ absl::StatusOr<std::shared_ptr<CrlProvider>> CreateStaticCrlProvider(
 void grpc_tls_credentials_options_set_crl_provider(
     grpc_tls_credentials_options* options,
     std::shared_ptr<grpc_core::experimental::CrlProvider> provider);
-
 #endif /* GRPC_GRPC_CRL_PROVIDER_H */