Add time to filesystem API

[earlephilhower]

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

[earlephilhower]

By removing the possibly data-corrupting changes required for SPIFFS time addition (see PR #6315), this PR should be able to be merged at 0 risk and pave the way for a future SPIFFS enhancement to include dynamic metadata and others (possibly w/the 3.0 partition table giving us the FS info as opposed to us manually poisoning things).

[d-a-v]

LGTM
I'd add a note stating stored time is always local time for LittleFS
(like it's always been in msdos fat but not in modern FSes).

[devyte]

I understand the FS is using local time stamps. If so, how about using global time instead, like real platforms? That would disallow problems with changing timezone, changing DST, updating time manually, etc.

[earlephilhower]

I understand the FS is using local time stamps. If so, how about using global time instead, like real platforms? That would disallow problems with changing timezone, changing DST, updating time manually, etc.

As we discussed online, this is what we're stuck with for SDFS/SD, since FAT doesn't use UTC, only local time. It is possible, however, for users to override the setTimeCallback() to return UTC if they wish for LittleFS (or even FAT) and documented in the File section.

[earlephilhower]

I couldn't find a good way to use the enum for FSId (each class' enum was a unique type so there were ugly casts), so I settled on a static constextr uint32_t member in each with the ID. I believe that's the proper way to do it for something like this, but I'm open to suggestions on other ways.

[hellochenwang]

where is the function time(NULL) referred in FS.h?

[d-a-v]

In tools/xtensa-lx106-elf/xtensa-lx106-elf/include/time.h
(this is newlib)