[9.x] Add a findOr method to Eloquent

[jessarcher]

This PR adds a findOr() method to the Eloquent Builder and Relations. This matches the existing firstOr() methods.

A typical scenario where this method is useful is when a resource route (e.g. /posts/1) cannot find a model that is not part of the URL, and so a findOrFail() returning a 404 is not appropriate.

Example usage:

User::findOr(1, fn () => throw new RuntimeException);
User::findOr(1, fn () => abort(403));
User::findOr(1, fn () => 'return something');

The method also supports passing columns as the second argument:

User::findOr(1, ['columns'], fn () => '...');

It also works on HasMany, HasManyThrough, BelongsToMany, MorphMany, and MorphToMany relations:

$user->posts()->findOr(1, fn () => '...');

[BrandonSurowiec]

Neat idea. Has some overlap use-case wise with Route::missing() that was added in 8.x: #36035

[mreduar]

Nice!

[novalramdhani]

Great feature!!!

[Technoyer]

This method adds some flexibility instead of findOrFail() we can now handle the fail output using whatever callback.
@jessarcher congratulations for the great work.