[8.x] Add method for on-demand log creation

[jobyh]

This patch proposes a new method LogManager::build to create on-demand logger instances.

The implementation and method name are based on the recently added FilesystemManager::build method.

use Illuminate\Support\Facades\Log;

$logger = Log::build([
    'driver' => 'single',
    'path' => storage_path('logs/custom.log'),
]);

$logger->info('Ahoy custom log!');

// Use in on-demand stack.
$stack = Log::stack(['syslog', $logger]);
$stack->warning('Watch out, stack!');

Why is this useful?
Occasionally it would be useful to configure a logger at runtime. For example setting the log path based on a command option or writing entries to their own location for a niche use-case or debugging.

This could also be useful to divert to a one-off log from artisan tinker.

I needed this functionality recently for a command to ingest several hundred models from an external XML data export. The XML was provided by a 3rd party and expected to contain a small number of errors which needed recording and communicating back for correction. To this end the command allows dry runs which do not modify the database but log results to a user-defined local path.

In order to write to a dedicated log file based on an optional parameter I ended up duplicating and adapting initialization code from within the core LogManager inside the command which felt a bit of a kludge. As use will be very infrequent adding an entry to the main config/logging.php seemed overkill.

Is it backwards compatible?
The new functionality extends the existing public API so should not be a breaking change.

[taylorotwell]

Can you explain why this change was necessary?

[jobyh]

The on-demand loggers are not resolvable by name using ->channel() as they do not have a name which is stored in config. This modification was to support using them in Log::stack() by including on-demand logger instance in the array alongside name strings of loggers from config/logging.php.

[jobyh]

I'll put together a PR adding this to the docs tomorrow