Support modifying the copied media assets in the build process

[emmadesilva]

An interesting case brought up at the Laravel Meetup in Malmö was using the asset build pipeline to manipulate images. Had an idea on how to make this even easier to hook into. We could instead of copying files in the build process, call a compile/similar method on the instance. The default implementation is to return a binary copy of the source file that we write to disk, but a developer could create a custom media class that could for example override this method to read the source file, compress it, and return the compressed file binary contents which we then write. We would need a way to support swapping out the class so I feel this is something we can do at a later time, but we should add the plumbing to support the root stuff here.

Originally posted by @caendesilva in #1904 (comment)

[emmadesilva]

This could pair nicley with deferred throwing #1904 (comment) to validate existence on metadata boot or copying, whichever is earliest.

Originally posted by @caendesilva in #1904 (comment)

[emmadesilva]

This was as simple as using put contents in 1a77854 since we already have a method to get the raw binary contents. So that could potentially be modified.

If this is a common thing people want, we can also add a way to add a post-processing/save-using callback to have a service provider binding to customize the process to hook in stuff like compression.

Originally posted by @caendesilva in #1904 (comment)

[emmadesilva]

Closing as preliminary completed by 1a77854 (so by extension fixed by #1904). The easiest way to do this for now is to customize this part of TransferMediaAssets.php:

$this->withProgressBar(MediaFile::all(), function (MediaFile $file): void {
    $sitePath = $file->getOutputPath();
    $this->needsParentDirectory($sitePath);
    Filesystem::putContents($sitePath, $file->getContents());
});

If people want this feature to have full support, ping me and we'll add a more robust solution!