Add `read_ixmp4()` method and extend integration test

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• ~Name of contributors Added to AUTHORS.rst~
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR adds an initial read_ixmp4() function to query data from an ixmp4 platform. Note that this function currently only allows to get all IAMC-timeseries data and meta indicators for all scenarios or only default scenarios. More elaborate filters will be added in a subsequent step.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.36842% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.8%. Comparing base (350d57a) to head (c2cf7a5).

Files	Patch %	Lines
pyam/ixmp4.py	93.3%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #826 +/- ## ===================================== Coverage 94.8% 94.8% ===================================== Files 64 64 Lines 6092 6125 +33 ===================================== + Hits 5778 5810 +32 - Misses 314 315 +1 ```

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