support all action types within composite actions

docs/adrs/0000-composite-actions.md

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+**Date**: 2021-04-06
+
+**Status**: Accepted
+
+## Context
+
+We will add support for actions to be referenced, with composite actions.
+
+## Decision
+
+TODO:
+  - Decide recursion limit
+
+## Consequences
+
+TODO: Move these details into a tracking issue instead
+
+- Launch
+  - New feature flag
+  - Move Action policy enforcement from workflow run, to resolve actions endpoint 
+- Runner
+  - New feature flag
+  - Download all nested actions
+    - Decide recursion limit
+    - src/Runner.Worker/ActionManager.cs
+    - Precursor: Remove feature flag DistributedTask.NewActionMetadata
+  - Update schema to support `uses` within a composite step
+    - src/Runner.Worker/action_yaml.json
+    - src/Sdk/DTPipelines/Pipelines/ObjectTemplating/PipelineTemplateConverter.cs
+  - Manage inputs/outputs between nested layers
+    - Update server to generate context names when empty
+  - Fix issue with `hashFiles` within a composite action
+    - https://github.com/actions/runner/issues/991
+  - Support actions with pre-step and post-step
+    - Investigate whether CompositeActionHandler.cs especially wrt clearing the output scopes
+    - Investigate whether we need additional validation to restrict contexts available to pre-step condition and post-step condition
+  - Testing
+    - Support all types of action manifests: Node.js, Dockerfile, `docker://`, composite, no manifest only Dockerfile
+    - Support all types of action references: `docker://`, `./`, owner/repo@ref
+    - Support for actions with pre-step and post-step