[feat] Support custom indexed and external references

[vkarak]

This PR extends the test's reference syntax to support arbitrary indexing on test attributes as well as other configuration parameters, such as the current partition, environment, processor and/or device details. If no special indexing is requested, the reference dictionary behaves exactly as before.

Support for external reference files is also added, where the reference dict of a test can be in an external YAML file.

Check the docs for all the details of the new features.

Implementation details

The test's prefix is now passed as a metaclass parameter and is not set during the test's initialization. This was needed to support the default external reference prefix, which coincides with the test's prefix, so we needed the test's prefix to be available during class creation and when the reference is defined in the test class body.

Closes #3463

[codecov]

Codecov Report

❌ Patch coverage is 95.85492% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.45%. Comparing base (998bf08) to head (b0ac411).

Files with missing lines	Patch %	Lines
reframe/core/pipeline.py	95.83%	7 Missing ⚠️
reframe/core/meta.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3630      +/-   ##
===========================================
+ Coverage    91.38%   91.45%   +0.06%     
===========================================
  Files           62       62              
  Lines        13520    13675     +155     
===========================================
+ Hits         12355    12506     +151     
- Misses        1165     1169       +4     

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