[feat] Add perflog_ignore attribute in httpjson handler

[ekouts]

Since Kibana rejects a record that includes a list of values of different types (for example check_perfvalues) I added the perflog_ignore in the configuration which will exclude these attributes from the json that we eventually send.
I don't think we should exclude the key from all logs. What do you think?

[ekouts]

@jenkins-cscs retry dom

[codecov-commenter]

Codecov Report

Base: 86.26% // Head: 86.23% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (f3e413c) compared to base (e27bc5d).
Patch coverage: 16.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2620      +/-   ##
==========================================
- Coverage   86.26%   86.23%   -0.03%     
==========================================
  Files          60       60              
  Lines       10962    10971       +9     
==========================================
+ Hits         9456     9461       +5     
- Misses       1506     1510       +4     

Impacted Files	Coverage Δ
reframe/frontend/cli.py	72.18% <ø> (ø)
reframe/core/logging.py	77.01% <16.66%> (-0.77%)	⬇️
reframe/core/containers.py	100.00% <0.00%> (ø)

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