MSC3283: Expose capabilities for profile actions (#3283)

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Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>
Co-authored-by: Will Hunt <will@half-shot.uk>
Co-authored-by: Dirk Klimpel <5740567+dklimpel@users.noreply.github.com>

proposals/3283-enable_set_displayname-capabilities.md

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+# MSC3283: Expose enable_set_displayname, enable_set_avatar_url and enable_3pid_changes in capabilities response 
+
+Some home servers like [Synapse](https://github.com/matrix-org/synapse/blob/756fd513dfaebddd28bf783eafa95b4505ce8745/docs/sample_config.yaml#L1207) 
+can be configured to `enable_set_displayname: false`, `enable_set_avatar_url: false` or `enable_3pid_changes: false`. 
+To enable clients to handle that gracefully in the UI this setting should be exposed.
+
+## Proposal
+
+The `/_matrix/client/r0/capabilities` endpoint should be decorated to provide more information on capabilities.
+```jsonc
+{
+  "capabilities": {
+    "m.set_displayname": { "enabled": false },
+    "m.set_avatar_url": { "enabled": false },
+    "m.3pid_changes": { "enabled": false },
+    "m.room_versions": {...},
+  }
+}
+```
+As part of this MSC, a capability for each setting will be added that exposes the server setting:
+- `m.set_displayname`
+
+Whether users are allowed to change their displayname after it has been initially set. 
+Useful when provisioning users based on the contents of a third-party directory.
+
+- `m.set_avatar_url`
+
+Whether users are allowed to change their avatar after it has been initially set. 
+Useful when provisioning users based on the contents of a third-party directory.
+
+- `m.3pid_changes`
+
+Whether users can change the 3PIDs associated with their accounts
+(email address and msisdn).
+Useful when provisioning users based on the contents of a third-party directory.
+
+## Client recommendations
+When presenting profile settings, clients should use capabilities in order to display the correct UI.
+
+Capability should always be present.
+Servers should always send these capabilities. If they aren't (because the server does not support 
+a new enough spec version or for any other reason), clients should behave as if they were present and set to true.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use `org.matrix.msc3283.` in place of `m.` throughout this proposal.