Declare httpClient variable.

[rosiel]

GitHub Issue: #975

• Other Relevant Links (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

What does this Pull Request do?

Removes a deprecation notice outlined in the above issue.

What's new?

Declare a variable that was being dynamically assigned.

How should this be tested?

Testing instructions to come.

Edit: Not sure how to make the errors show up, but an IDE (the built-in one in site-template) shows the lack of declaration of this variable as a problem.

Documentation Status

• Does this change existing behaviour that's currently documented? no
• Does this change require new pages or sections of documentation? no
• Who does this need to be documented for? nobody
• Associated documentation pull request(s): ___ or documentation issue ___

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

@alxp, @Islandora/committers

[rosiel]

Edit: This is a deprecation as of PHP 8.2. https://www.php.net/manual/en/migration82.deprecated.php#migration82.deprecated.core.dynamic-properties