firmware_handler: extract part responsible for running ext program to lib

As a preparation for upcoming commit, extract part which is responsible
for external program execution to libmodprobe so it can be later
re-used not only for firmware handler but also for dynamic module
options handler within libmodprobe.

RunExternalHandler is moved entirely to separate library with two
changes:
- Setting env needed previously by firmware handler was made generic and
  now external handler can get envs_map containing pairs of env and its
  value which needs to be set.
- "Firmware" was removed from one of the log since now it can be used in
  different context (s/External Firmware Handler/External Handler/)

Bug: 335619610
Test: `atest CtsInitTestCases` passed, especially:
      [105/129] ueventd_parser#ExternalFirmwareHandlers: PASSED (0ms)
      [106/129] ueventd_parser#ExternalFirmwareHandlersDuplicate: PASSED (0ms)

Change-Id: Ie07cee763278f224bd3c0acfbe06c44eb36d0a81