Telemetry:add app-client-configuration-change event

[marcotc]

This PR adds the telemetry event app-client-configuration-change, which reports that configuration at the Client Library (ddtrace) has changed.
This event captures the changes themselves and a change source (called origin in the event).

The first use case is reporting changes from Dynamic Configuration. These have the origin remote_config.
The changes are a list of 2-tuples (List<String, Integer | Boolean | String>) representing the configuration that was just set.

One thing to noticed is that app-client-configuration-change is a Telemetry V2 event, but ddtrace currently implements Telemetry V1.
To aid with the migration to V2, the Datadog backend supports a few V2 events through the V1 pipeline. This is one of them.

This PR does not actually report the new event, only creates the API to trigger this event.

This event is crucial to Dynamic Configuration because it reports back to Datadog that the changes have been applied successfully to the current Ruby process.

[GustavoCaso]

Left a few comments but overall LGTM

[marcotc]

Thank you, @GustavoCaso!

[codecov-commenter]

Codecov Report

Merging #2977 (2e71557) into master (544615c) will decrease coverage by 0.01%.
The diff coverage is 98.13%.

@@            Coverage Diff             @@
##           master    #2977      +/-   ##
==========================================
- Coverage   98.09%   98.08%   -0.01%     
==========================================
  Files        1301     1305       +4     
  Lines       72574    72672      +98     
  Branches     3365     3368       +3     
==========================================
+ Hits        71189    71283      +94     
- Misses       1385     1389       +4     

Impacted Files	Coverage Δ
spec/datadog/core/telemetry/client_spec.rb	95.32% <91.66%> (-0.60%)	⬇️
lib/datadog/core/telemetry/client.rb	93.18% <100.00%> (+0.49%)	⬆️
lib/datadog/core/telemetry/emitter.rb	100.00% <100.00%> (ø)
lib/datadog/core/telemetry/event.rb	100.00% <100.00%> (ø)
...re/telemetry/v2/app_client_configuration_change.rb	100.00% <100.00%> (ø)
lib/datadog/core/telemetry/v2/request.rb	100.00% <100.00%> (ø)
spec/datadog/core/telemetry/emitter_spec.rb	100.00% <100.00%> (ø)
spec/datadog/core/telemetry/event_spec.rb	100.00% <100.00%> (ø)
...lemetry/v2/app_client_configuration_change_spec.rb	100.00% <100.00%> (ø)
spec/datadog/core/telemetry/v2/request_spec.rb	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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