RUST-2174 feat(extjson): parse legacy {"$date": <ms since epoch>}

[xiangjinwu]

According to the specification, there exists a legacy Extended JSON form of Datetime
https://github.com/mongodb/specifications/blob/master/source/extended-json/extended-json.md#datetime

Mongoexport 2.4 and the Java driver always transform a Datetime object into an Extended JSON string of the form {"$date": <ms since epoch>}. This form has the problem of a potential loss of precision or range on the Datetimes that can be represented.

Specially, this legacy form is called JsonMode.STRICT in the Java JsonWriterSettings, while the canonical form is JsonMode.EXTENDED. The former is still used by the Debezium connector for MongoDB.

This PR adds the ability to parse such legacy format, without conflicting with the standard ones.

[abr-egn]

Thank you for the contribution! The CI failures were unrelated and this was a straightforward addition, so I've merged it in and it'll go out in the next release (should happen in the next week or so).