feat(AMQP): Add Header constants for consistent AMQP header usage

[butschster]

We're excited to introduce a new utility class for users of the AMQP driver! With the primary goal of improving readability and ensuring consistent usage of AMQP headers, we've added a dedicated class to provide a list of available header constants.

💡 Usage Example
The new header can be used to set the ROUTING KEY header as shown:

use Spiral\RoadRunner\Jobs\Queue\AMQP\Header;

/** @var QueueInterface $queue */
$queue->push(
    JobPayload::class,
    $payload,
    (new Options())
        ->onQueue('my_amqp_pipeline')
        ->withHeader(Header::ROUTING_KEY, '...') 
        ->withDelay(20)
);

roadrunner-server/roadrunner#1555
roadrunner-php/issues#7

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.15% ⚠️

Comparison is base (9185d6c) 86.52% compared to head (9bfc228) 86.37%.

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Additional details and impacted files

@@             Coverage Diff              @@
##                4.x      #58      +/-   ##
============================================
- Coverage     86.52%   86.37%   -0.15%     
  Complexity      191      191              
============================================
  Files            29       30       +1     
  Lines           579      580       +1     
============================================
  Hits            501      501              
- Misses           78       79       +1     

Files Changed	Coverage Δ
src/Queue/AMQP/Header.php	0.00% <0.00%> (ø)

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