update(libsinsp): get_dynamic_field constness

[deepskyblue86]

What type of PR is this?

/kind feature

Any specific area of the project related to this PR?

/area libsinsp

Does this PR require a change in the driver versions?

What this PR does / why we need it: Have get_dynamic_field being const, so it can be used as a regular getter.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[poiana]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.35%. Comparing base (c4ea330) to head (ee14b3e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2511   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files         281      281           
  Lines       31689    31689           
  Branches     4643     4643           
=======================================
+ Hits        24830    24831    +1     
+ Misses       6859     6858    -1     

Flag	Coverage Δ
libsinsp	78.35% <100.00%> (+<0.01%)	:arrow_up:

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[FedeDP]

/milestone 0.22.0

[leogr]

/milestone 0.23.0