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>

include/libflashrom.h

@@ -193,6 +193,14 @@ struct flashrom_chipset_info {
  * @return flashrom version
  */
 const char *flashrom_version_info(void);
+/**
+ * @brief Returns list of supported programmers
+ * The last entry in the returned list is followed by a NULL.
+ *
+ * @return List of supported programmers, or NULL if an error occurred.
+ *  The pointer must be freed after using flashrom_data_free
+ */
+const char **flashrom_supported_programmers(void);
 /**
  * @brief Returns list of supported flash chips
  * @return List of supported flash chips, or NULL if an error occurred