Add extension placeholder for serialized medias

[Prokyonn]

@alexander-schranz @wachterjohannes to prevent the BC break, we could maybe add a config like this:

sulu_headless:
    media:
        use_preferred_extension: true

WDYT?

[alexander-schranz]

@Prokyonn if somebody is using ^0.9 they will not get automatically 0.10, as ^ in pre stable versions is handled like 0.9.* so active update is required and so I'm fine with this bc break and no additional flag.

[wachterjohannes]

@alexander-schranz @wachterjohannes to prevent the BC break, we could maybe add a config like this:

sulu_headless:
    media:
        use_preferred_extension: true

WDYT?

@Prokyonn would also go with a flag and throw a deprecation message when it is not set! else the PR LGTM.

[Prokyonn]

as discussed internally, we'll omit the configuration BC layer, because of

if somebody is using ^0.9 they will not get automatically 0.10, as ^ in pre stable versions is handled like 0.9.* so active update is required and so I'm fine with this bc break and no additional flag.