Add time to filesystem API (#6544)

* Add time to filesystem API

Support the ESP32 File::getLastWrite() call and setting the time on
all filesystems automatically (assuming the system clock has
been set properly and time(NULL) returns the proper time!).

Adds Dir::fileTime() to get the time of a file being listed, similar to
Dir::fileName() and Dir::fileSize().

Adds ::setTimeCallback(time_t (*cb)()) to File, Dir, and FS, allowing
users to override the default timestamp on a per-file, directory, or
filesystem basis. By default, a simple callback returning time(nullptr)
is implemented.

LittleFS uses the 't' attribute and should be backwards compatible.

SD/SDFS work and include wrappers for obsolete SdFat timestamp callbacks
using the MSDOS time.

This PR does not update SPIFFS, due to compatability concerns and a
possible massive rewrite which would make it possible to determine if an
old-style ot metadata enabled FS is present at mount time.

Includes an updated SD/listfiles and LittleFS_time example.

Replaces #6315

* Add links to new mklittlefs w/timestamp support

Include the update mklittlefs which generated 't' metadata on imported
files.
	../tools/sdk/lwip2/include/netif/lowpan6_opts.h

* Add explicit note about timestamp being local time

* Address review concerns

Clean up some awkward object instantiations.

Remove the _enableTime flag/setter from SPIFFS.

Clean up the FSConfig constructors using C++ style init lists.

libraries/SD/src/SD.cpp

@@ -3,3 +3,5 @@
 #if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_SD)
 SDClass SD;
 #endif
+
+void (*__SD__userDateTimeCB)(uint16_t*, uint16_t*) = nullptr;