spiffs auto format selector (true by default)

[d-a-v]

To help debugging #2581 (wemos/lolin d1 mini pro and/or 16MB flash):
ESP.setAutoFormatSPIFFS() will not format SPIFFS when mount fails.

Helpers set in ESP:: because this is the most simple way.
SPIFFS real interface is hard to reach from userland.

[d-a-v]

To the question: why not SPIFFS.begin((autoFormat=)false), the answer is:
SPIFFS's type is not SPIFFSImpl:: but FS:: and this is a bag of bones.

This PR is mainly here to help debugging.
We can remove that once we have reliable 16MB SPIFFS.
Or rewrite it when FS (sd, spiffs) is refactored.

[earlephilhower]

@d-a-v, I've got the same issue. Maybe if we did something like make a per-FS configuration object?

class FSConfig {
public:
  FSConfig();
}; // empty base

class SPIFFSConfig : public FSConfig {
public:
 SPIFFSConfig(bool autoFormat = false) {_autoFormat = autoFormat; }
 void disableAutoFormat() { _autoformat = false; }
 bool _autoFormat;
};

class SDFSConfig : public FSConfig {
public:
  SDFSConfig(int cs = 4, SPISettings spi =SPI_MHZ(10)) {
    _cs = cs; _spi = spi;
   }
  int _cs;
  SPISettings spi;
};

Then the FSImpl object begin signature becomes:
begin( FSConfig &cfg = nullptr );

SPIFFS::begin can then static_cast its pointer to SPIFFSConfig, etc.

User code then becomes
SPIFFS.begin() or SPIFFS.begin( SPIFFSConfig(false) ); or SPIFFSConfig cfg; cfg.disableAutoFormat(); SPIFFS.begin(cfg);

[d-a-v]

This PR was a proposal,
You are currently mastering FSImpl with new SD, LittleFS, with SPIFFS.
I was thinking of adding a virtual autoformat FSImpl call but @devyte, or you might think differently.
I'm not hitting the Close&Comment button right now, but my finger is burning.
Please do your magic!

[earlephilhower]

@d-a-v, take a look at #5525 which is my proposal for this (SPIFFS and SDFS now, LittleFS once one or the other merges).

[d-a-v]

closed in favor of #5525