[package] ffmpeg: add option to use the opencore amr nb/wb decoder instead of built-in amr decoder

[hwse]

The internal ffmpeg amr wideband/narrowband decoder lacks some features and suggests to use opencore-amr (see here: https://github.com/FFmpeg/FFmpeg/blob/8272d34377b24e52de0492fbc2e1d74efcdff248/libavcodec/amrnbdec.c#L989C47-L989C64 ).

There is already a package for opencore-amr, so only the ffmpeg package needs a option to enable it.

Related Pull Request: #26217

Note that using opencore-amr (and some other libraries) changes the ffmpeg license to (L)GPLv3 (from v2).

for licensing see here: https://github.com/FFmpeg/FFmpeg/blob/8272d34377b24e52de0492fbc2e1d74efcdff248/LICENSE.md?plain=1#L110