[12.x] Add ability to prepend() and append() to PendingChain

[stevebauman]

Description:

This PR adds the ability to prepend and append jobs on a Bus PendingChain, giving developers the ability to conditionally transform the chain before its dispatched.

Example:

$recording = Media::first();

$chain = Bus::chain([
    new TranscribeRecording($recording)
]);

if ($this->hasAudioNormalizationEnabled()) {
    $chain->prepend(new NormalizeAudio($recording));
}

if ($this->hasTranscriptionRedactionEnabled()) {
    $chain->append(new RedactTranscription($recording));
}

$chain->dispatch();

In the example above:

• If audio normalization is enabled, the NormalizeAudio job will be moved to the first position of the chain and the TranscribeRecording job will be moved to the second
• If transcription redaction is enabled, it will be pushed onto the back of the chain ensuring it is always the last job to run regardless of whichever ones preceded it

Let me know your thoughts! Thanks for your time 🙏

[stevebauman]

This was intentionally changed so developers can make chains without any initial argument:

$chain = Bus::chain();

$chain->append(...);

$chain->prepend(...);

$chain->dispatch();

If you'd rather avoid this, let me know!

[shaedrich]

Funny how one can be so trained to see something, that one instantly goes

as in this case: when() 😅

Bus::chain([
    new TranscribeRecording($recording)
])->when(
    $this->hasAudioNormalizationEnabled(),
    fn ($chain) => $chain->prepend(new NormalizeAudio($recording)),
)->when(
    $this->hasTranscriptionRedactionEnabled(),
    fn ($chain) => $chain->append(new RedactTranscription($recording)),
)->dispatch();