Support nested projection in get_projection

[ryotatomioka]

Currently, bunnet.odm.utils.projection.get_projection does not support nested projection model. Although I only have experience with Mongo DB backed up by Azure Cosmos DB, in my experience this makes a huge difference (~50-100x) in query speed when the size of the field to be projected is much smaller (e.g., a single string) compared to the hierarchy of models containing it.

For example,

from bunnet.odm.utils.projection import get_projection

class Level2(BaseModel):
    id: str

class Level1(BaseModel):
    level_2: Level2

class Level0(BaseModel):
    level_1: Level1

d = get_projection(Level0)  # returns {"level_1": 1}

The following simple change fixes this (returns {"level_1.level_2.id": 1})

diff --git a/bunnet/odm/utils/projection.py b/bunnet/odm/utils/projection.py
index 79eb59e..408b0b2 100644
     for name, field in get_model_fields(model).items():
-        document_projection[field.alias or name] = 1
+        if issubclass(field.annotation, BaseModel):
+            inner_projection = get_projection(field.annotation)
+            for key, val in inner_projection.items():
+                document_projection[f"{name}.{key}"] = val
+        else:
+            document_projection[field.alias or name] = 1
     return document_projection