Hook up custom timestamp proc for SD/SDFS (#7686)

The SDFS implementation didn't include plumbing to support user-supplied
timestamp generators (for file create/write times) because the SDFat
library doesn't support a callback parameter.

Work around it by using a single, global (inside sdfs namespace) that
the static timestamp callback member can see and use.

Add a test of the feature in CI.

Fixes #7682

libraries/SDFS/src/SDFS.cpp

@@ -37,6 +37,8 @@ FS SDFS = FS(FSImplPtr(new sdfs::SDFSImpl()));
 
 namespace sdfs {
 
+// Required to be global because SDFAT doesn't allow a this pointer in it's own time call
+time_t (*__sdfs_timeCallback)(void) = nullptr;
 
 FileImplPtr SDFSImpl::open(const char* path, OpenMode openMode, AccessMode accessMode)
 {