rename core configuration option on_set to after_set

[GustavoCaso]

What does this PR do?

Rename the configuration on_set to after_set to better reflect the nature of it. It is a callback executed after the value has been set.

Motivation:

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

Merging #3107 (26ad90f) into master (f2c8241) will decrease coverage by 0.01%.
Report is 16 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3107      +/-   ##
==========================================
- Coverage   98.16%   98.15%   -0.01%     
==========================================
  Files        1323     1323              
  Lines       75155    75155              
  Branches     3428     3428              
==========================================
- Hits        73773    73771       -2     
- Misses       1382     1384       +2     

Files Changed	Coverage Δ
lib/datadog/tracing/distributed/propagation.rb	92.10% <ø> (ø)
lib/datadog/core/configuration/option.rb	100.00% <100.00%> (ø)
...ib/datadog/core/configuration/option_definition.rb	98.85% <100.00%> (ø)
lib/datadog/core/configuration/settings.rb	98.47% <100.00%> (ø)
lib/datadog/tracing/configuration/settings.rb	94.92% <100.00%> (ø)
...cing/contrib/action_pack/configuration/settings.rb	96.29% <100.00%> (ø)
...og/tracing/contrib/rails/configuration/settings.rb	97.67% <100.00%> (ø)
...dog/tracing/contrib/rake/configuration/settings.rb	100.00% <100.00%> (ø)
...tadog/core/configuration/option_definition_spec.rb	100.00% <100.00%> (ø)
spec/datadog/core/configuration/option_spec.rb	99.62% <100.00%> (ø)

... and 2 files with indirect coverage changes

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