Configuration: Allow customizing through functional opts

[alvaroaleman]

Currently, any customization to the default config is very very awkward to do:

cfg := datadog.DefaultConfiguration()
cfg.Debug = true

client := datadog.NewAPIClient(cfg)

with this change, functional opts can be used to simplify that:

client := datadog.NewAPIClient(datadog.DefaultConfiguration(func(cfg *datadog.Configuration){
    cfg.Debug = true
}))

What does this PR do?

Additional Notes

Review checklist

Please check relevant items below:

• This PR includes all newly recorded cassettes for any modified tests.

• This PR does not rely on API client schema changes.

  • The CI should be fully passing.

• Or, this PR relies on API schema changes and this is a Draft PR to include tests for that new functionality.

  • Note: CI shouldn't be run on this Draft PR, as its expected to fail without the corresponding schema changes.

[alvaroaleman]

@HantingZhang2 @nkzou any chance you could have a look, please?

[github-actions]

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