Add changelog entry

Author: agustinmista

ouroboros-consensus-diffusion/changelog.d/20251126_161528_agustin.mista_introduce_feature_flags.md

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+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+<!--
+### Patch
+
+- A bullet item for the Patch category.
+
+-->
+<!--
+### Non-Breaking
+
+- A bullet item for the Non-Breaking category.
+
+-->
+### Breaking
+
+- Add `rnFeatureFlags` field to `RunNodeArgs` to store enabled experimental feature flags.
+