Deprecate AbstractApi::get()

[Art4]

AbstractApi::get() does not allow to specify the content type header for the request and could return string|array|SimpleXMLElement|false. In this PR I moved the logic from this method into the API classes. There we can better see the possible return types and I've updated the phpdoc return types accordingly.

Thanks to new and improved tests I could also fix a bug in Redmine\Api\Attachment::download() returning false on error instead of the HTML 404 error page.

This PR removes the @internal mark from the new HttpClient and Request interfaces and deprecates AbstractApi::get().

I will do the same for AbstractApi::post(), AbstractApi::put() and AbstractApi::delete() in separate PRs, see #372.

Requires #378.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (81259f3) 97.63% compared to head (2f6b3d7) 97.71%.

Additional details and impacted files

@@             Coverage Diff              @@
##               v2.x     #381      +/-   ##
============================================
+ Coverage     97.63%   97.71%   +0.07%     
- Complexity      563      589      +26     
============================================
  Files            29       29              
  Lines          1609     1707      +98     
============================================
+ Hits           1571     1668      +97     
- Misses           38       39       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[gitguardian]

⚠️ GitGuardian has uncovered 2 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
9556153	Triggered	Generic High Entropy Secret	97b80d9	tests/Behat/features/wiki.feature	View secret
9556153	Triggered	Generic High Entropy Secret	97b80d9	tests/Behat/features/wiki.feature	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secrets safely. Learn here the best practices.
3. Revoke and rotate these secrets.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

---

<sup>🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.

Our GitHub checks need improvements? Share your feedbacks!</sup>