Add RV as a participant using the new Live Review schema (AcademySoftwareFoundation#602)

### Add RV as a participant using the new Live Review schema

### Summarize your change.

The `read_otio_string` function was added to parse the OTIO object
containing the timeline sent by the Review App when RV is joining a new
session as a participant. Since the timeline contains some annotation
fields, the annotation schema was added in order to let OTIO know that
it is a known schema during the deserialization step. However, nothing
is done with the annotations at this stage.

### Describe the reason for the change.

This is the first step in adding back the new OTIO schema as the
preferred way of communication between RV and the Review App.

### Describe what you have tested and on which operating system.

Live review using the new schema was tested between a local Review App
where the feature was enabled with
https://creative-collab.dev.shotguncloud.com and RV on MacOS.

---------

Signed-off-by: Éloïse Brosseau <[email protected]>
Signed-off-by: Cédrik Fuoco <[email protected]>

Author: eloisebrosseau

src/plugins/rv-packages/otio_reader/otio_reader.py

@@ -60,6 +60,21 @@ class NoNodeFromHook(otio.exceptions.OTIOError):
     pass
 
 
+def read_otio_string(otio_string: str, host_prefix: str | None = None) -> object | None:
+    """
+    Main entry point to expand a given otio string into the current RV session.
+
+    Returns the top level node created that represents this otio
+    timeline.
+    """
+    otio_obj = otio.adapters.read_from_string(otio_string)
+    timeline = otio_obj["otio"]
+
+    context = {"sg_url": host_prefix} if host_prefix else None
+
+    return create_rv_node_from_otio(timeline, context), timeline.global_start_time
+
+
 def read_otio_file(otio_file):
     """
     Main entry point to expand a given otio (or file otio can read)