Fixing _rawBody handling and adding option to maintain _ts and _etag properties from origin

[FabianMeiswinkel]

Description

When reading a DataFrame from Cosmos without schema inference you get a property _rawBody which contains the json of the payload. It should be possible to write this Dataframe back to Cosmso - the _rawBody property is supposed to be used as the new payload (but system properties like _ts and _etag would be overridden in the backend). There was a bug that missed triggering the special casing of _rawBody in certain cases (when _rawBody was not nullable) and it was just written as a json property "_rawBody" : "xxx"
This PR fixes that.
The PR also adds a new capability - if the Dataframe written to Cosmos contains a column "_origin_rawBody" instead of "_rawBody" it gets in general handled like "_rawBody" - but it iwll preserve the _ts as _origin_ts and _etag as _origin_etag. This fills a common gap when trying to identify "missing" or "not yet migrated" records when copying from a source container to a target container.

So, to simplify this scenario customer would just

• Read dataframe form source container without schema inference
• Rename column _rawBody to _origin_rawBody
• Write that DF to target container

Then a join could be used to identify records in source where the target container doesn't contain the same version (or a later version)

Testing Guidelines

• Pull request includes test coverage for the included changes.

[RaviTella]

Looks good! I was able to do a quick test to.
Source:
{
"id": "4",
"category": "Programming Languages",
"title": "Learning Concurrent Programming in Scala",
"_rid": "WrMCAOg+m1UPAAAAAAAAAA==",
"_self": "dbs/WrMCAA==/colls/WrMCAOg+m1U=/docs/WrMCAOg+m1UPAAAAAAAAAA==/",
"_etag": ""0801e424-0000-0700-0000-61eed7410000"",
"_attachments": "attachments/",
"_ts": 1643042625
}

Sink
{
"id": "4",
"category": "Programming Languages",
"title": "Learning Concurrent Programming in Scala",
"_rid": "WrMCAJKKoDEMAAAAAAAAAA==",
"_self": "dbs/WrMCAA==/colls/WrMCAJKKoDE=/docs/WrMCAJKKoDEMAAAAAAAAAA==/",
"_etag": ""a801b681-0000-0700-0000-61fad25b0000"",
"_attachments": "attachments/",
"_lsn": 146,
"_origin_etag": ""0801e424-0000-0700-0000-61eed7410000"",
"_origin_ts": 1643042625,
"_ts": 1643827803
}

[ealsur]

Thanks for the new tests!

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