Improvement: Add defaults and settings validation to `FFmpegConfigManager`

[omeryusufyagci]

Currently the FFmpegConfigManager provides an interface for the FFmpeg-specific configurations and user settings.
However, it should also provide a set of sensible defaults for ease of use, and it validate each set operation performed on it.
This simplifies the process for FFmpeg users by eliminating concerns about potentially inter-dependent validations of FFmpeg settings. For more context, see #51.

[andcscott]

Hey @omeryusufyagci, wanted to check if the revisions for #60 (commit 467fe76 on my branch) took care of this? If any refactoring is needed I'll have some time this week/next week if you'd like.

[omeryusufyagci]

Hey @andcscott, thanks for checking in! Yes, it has addressed it partially. Now we have defaults which is great, but I also want to ensure the config manager is entirely responsible for the validation of the fields as well. There are cases where some codecs won't work with certain other settings and of course the basic validation of each field to check if they're within bounds, and are logical to a point, etc.

If you'd like to take a pass at it, I'd gladly accept the help ! : )