Add support for validating multiple messages and registration tokens

[jeromegamez]

Addresses #511

With this, it will be possible to validate up to 500 messages and/or registration tokens for validity at once.

How to test

composer require "kreait/firebase-php: dev-multi-validate"

Validate registration tokens

$registrationTokens = [/* ... */];

$result = $messaging->validateRegistrationTokens($registrationTokens);

The result will be an array with the following format:

[
    'valid' => [/* valid tokens that are valid and registered to the current Firebase project */],
    'unknown => [/* valid tokens that are valid, but not registered to the current Firebase project */],
    'invalid' => [/* invalid/malformed tokens*/],
]

Validate multiple messages

Validating multiple messages has the same limitations as actually sending them (only 500 messages can be checked at a time - if you need to validate more, you need to chunk the items beforehand)

$validateOnly = true;

$messaging->send($message, $validateOnly);
$messaging->sendMulticast($message, $tokens, $validateOnly);
$messaging->sendAll($messages, $validateOnly);

[codecov]

Codecov Report

Merging #520 (098efff) into 5.x (d637e80) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                5.x     #520      +/-   ##
============================================
+ Coverage     92.60%   92.73%   +0.13%     
  Complexity     1543     1543              
============================================
  Files           152      151       -1     
  Lines          3744     3744              
============================================
+ Hits           3467     3472       +5     
+ Misses          277      272       -5