add Datadog Trace Export Configuration

[ericmustin]

Description:
This PR adds Trace export configuration to the Datadog exporter overall structure, following the contributing guidelines.

This adds the configuration necessary to then add the ability to export traces directly to Datadog's backend (agentless).

After the configuration is confirmed we plan to make additional PRs to bring in the actual bulk code for exporting traces. Contribution guidelines state we should break this up, but you can find this work in progress (though functional) here, on the our fork, for context.

You can also find additional work for metric export code in this pr. I don't think these are tightly coupled but happy to rebase if one gets merged before the other.

Any feedback or comments here are welcome and we'll try to prioritise, as well as if you have any feedback on in progress work on our fork. I'm also available on gitter for anything ad-hoc. We're keen to get both the metric and trace code into the collector and support it however necessary.

Cheers, Eric

Link to tracking Issue:

Testing: Added unit tests, tested full version (available on our fork) on an end to end testing environment.

Documentation: This adds the README for the exporter with the intended instructions for the full trace exporter.

[codecov]

Codecov Report

Merging #1181 into master will increase coverage by 13.95%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #1181       +/-   ##
===========================================
+ Coverage   76.01%   89.96%   +13.95%     
===========================================
  Files          24      283      +259     
  Lines        1163    13782    +12619     
===========================================
+ Hits          884    12399    +11515     
- Misses        226     1014      +788     
- Partials       53      369      +316     

Flag	Coverage Δ
#integration	76.09% <ø> (+0.08%)	⬆️
#unit	89.12% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/datadogexporter/config.go	92.30% <100.00%> (ø)
exporter/datadogexporter/factory.go	100.00% <100.00%> (ø)
exporter/sentryexporter/utils.go	100.00% <0.00%> (ø)
exporter/awsxrayexporter/factory.go	100.00% <0.00%> (ø)
receiver/k8sclusterreceiver/collection/replica.go	100.00% <0.00%> (ø)
...stransformprocessor/metrics_transform_processor.go	100.00% <0.00%> (ø)
extension/observer/endpointswatcher.go	93.75% <0.00%> (ø)
exporter/azuremonitorexporter/conventions.go	100.00% <0.00%> (ø)
...sformprocessor/operation_toggle_scalar_datatype.go	64.28% <0.00%> (ø)
...sreceiver/awsecscontainermetrics/stats_provider.go	100.00% <0.00%> (ø)
... and 265 more

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