Use of external flash memory

[mikee47]

At present the library is limited to storing all its data in PROGMEM which, on the ESP8266, is limited to 1MB. Large BLOBs would ideally be stored in a separate flash partition.

For example:

// Length of the data
constexpr size_t stringLength = 85535;
// Address of data within the flash
constexpr uint32_t stringAddress = 0x200000;
DEFINE_FSTR_EXT(myExternalString, stringLength, stringAddress);

The idea would be that the length is stored in PROGMEM, but the data is external.