libflashrom: Add API to get the list of supported programmers

There were no options available to obtain the list of programmers.
The implementation is based on flashrom_supported_flash_chips.
Arrays of constant strings are returned, and the array must be
freed using flashrom_data_free.

Testing: Both unit tests and CLI tools serve as libflashrom clients.
    All unit tests run successfully.

Change-Id: Ib5275b742b849183b1fe701900040fee369a1d78
Signed-off-by: Dmitry Zhadinets <dzhadinets@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/86921
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

libflashrom.map

@@ -32,6 +32,7 @@ LIBFLASHROM_1.0 {
     flashrom_supported_boards;
     flashrom_supported_chipsets;
     flashrom_supported_flash_chips;
+    flashrom_supported_programmers;
     flashrom_version_info;
     flashrom_wp_cfg_new;
     flashrom_wp_cfg_release;