Naming of SPAR "FOA" functions/files (193)

The names of several SPAR source files, structures, and functions mention 'FOA' which is not always right and thus confusing. It is thus proposed to get rid of the FOA part in the SPAR names as the name does not correspond to the actual content.

E.g.

1. Move content of ivas_spar_foa_enc.c to ivas_spar_enc.c.
2. Rename ivas_spar_foa_md_enc.c to ivas_spar_md_enc.c.
3. Similarly for functions, structures, etc.

The same applies to the decoder.