docs: adds ADR 13. Clickhouse experimental and beta features (#113)

docs/decisions/0013_clickhouse_experimental.rst

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+13. Clickhouse experimental and beta features
+#############################################
+
+Status
+******
+
+Accepted
+
+Context
+*******
+
+When the Aspects project started, Clickhouse was working on a JSON column type and expected to have it out of
+"experimental" status by Sep 2023. However, Clickhouse revealed that the feature development is stuck in limbo, and may
+never be completed.
+
+Clickhouse documentation warns that `experimental features`_ may never become accepted into the general feature list,
+may be removed at any time, and are not supported by the Clickhouse development team. Additionally, "experimental"
+features may not be available on all Clickhouse hosting providers.
+
+By contrast, `beta features`_ are on track to becoming generally-available features, and so though they may change, they
+are fully supported by the Clickhouse team, and will eventually be available on all Clickhouse hosting providers.
+
+Decision
+********
+
+**Avoid experimental features**
+
+Find alternative solutions and avoid enabling experimental features in Clickhouse.
+
+**Use beta features with caution**
+
+Use Clickhouse `beta features`_ only if no alternative solution can be found.
+
+Plan a feature flag to disable functionality that depends on beta features, to avoid breaking deployments where
+Clickhouse beta features are not available.
+
+Consequences
+************
+
+#. Remove JSON field types from Clickhouse table schemas, and remove references to the flag that enables them
+   (``allow_experimental_object_type``).
+
+Rejected Alternatives
+*********************
+
+None
+
+References
+**********
+
+* `tutor-contrib-aspects#376`_ Feat: Remove JSON column from xapi_events_all
+
+.. _beta features: https://clickhouse.com/docs/en/beta-and-experimental-features#beta-features
+.. _experimental features: https://clickhouse.com/docs/en/beta-and-experimental-features#experimental-features
+.. _tutor-contrib-aspects#376: https://github.com/openedx/tutor-contrib-aspects/issues/376