Add support for Vector field type

[JaredHatfield]

Firestore has introduced a new field type of "Vector" which is Pre-GA at this time. This new field type is not supported by firestoreproto2map and therefore won't be supported for generating the map object until it is added.

https://cloud.google.com/firestore/docs/concepts/data-types

[JaredHatfield]

Some research was done on this topic and documented UnitVectorY-Labs/firestoreproto2json#21 (comment)

The relevant point for firestoreproto2map is that the vector object representation is actually already stored as a Map in the Firestore document for the underlying protocol buffers.

While the Python and Node.JS library have been enhanced to this as a Map (see the relevant code snippet in the node.js library where this map is handled as a special case https://github.com/googleapis/nodejs-firestore/blob/392ecf37d3ead79b90e5bbe9e54ca677a026c525/dev/src/convert.ts#L166) this poses challenges for how this should be handled here.

There is no Java object Google has created at this time to represent a vector. Therefore the current implementation converts these into a map successfully without any changes.

The best approach here is to leave this code unchanged at this time as the goal is to have compatibility with the Map so it can be inserted into Firestore and maintain compatibility.

The next step is that: manually insert a map into Firestore that matches this and see if the Firestore console shows that it is in fact being treated as a Vector or is it somehow treated as a Map that matches the exact structure of a Vector.