Integrations field DSL: Add support for feature flags

This issue came out of this discussion !90921 (comment 1009216220).

Problem

Using a feature flag to toggle the visibility of fields leads to bespoke solutions like the overriding of Datadog#fields !90921 (comment 1009216220).

Solution

We could extend Integrations::Field to support an :if attribute. The value would be a proc. Integration#fields could filter out fields where the proc evaluates to false (passing in the integration as an argument). We would update relevant developer docs.