[feat] Add metadata to environment definitions in the configuration

[vkarak]

This is the complete analogous to the extras in the definition of the partitions. Users can now define an extras attribute in their environment configuration, which allows them to attach arbitrary properties to environments. These are accessible inside a test through the extras attribute of the current_environ.

Fixes #1973.
Fixes #2230.

[vkarak]

@jenkins-cscs retry dom

[codecov-commenter]

Codecov Report

Merging #2265 (a1c0e48) into master (81f1415) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head a1c0e48 differs from pull request most recent head 3e524f8. Consider uploading reports for the commit 3e524f8 to get more accurate results

@@           Coverage Diff           @@
##           master    #2265   +/-   ##
=======================================
  Coverage   86.12%   86.13%           
=======================================
  Files          56       56           
  Lines       10198    10202    +4     
=======================================
+ Hits         8783     8787    +4     
  Misses       1415     1415           

Impacted Files	Coverage Δ
reframe/core/systems.py	89.47% <ø> (ø)
reframe/core/environments.py	97.19% <100.00%> (+0.10%)	⬆️

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