Support for flat vertex property format added to Java version

[scgbear]

This issue is for a: (mark with an x)

- [ ] bug report -> please search issues before submitting
- [ X ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

Expected/desired behavior

Add support for flat vertex property format. This is where vertex properties are written as key : scalar_value in json as opposed to key : array_of_objects.
In dotnet it is called the flag which determines this is called isMultiValuedAndMetaPropertiesDisabled, though I feel that this is a little obtuse and instead we should go for enableFlatVertexPropertySchema.

Mention any other details that might be useful

This could be done by implementing a second objectMapper for the different serialization approach. The BulkGremlinObjectMapper utility class could be extended to include a getFlatVertexObjectMapper renaming the existing getBulkGremlinObjectMapper to something like getArrayVertexObjectMapper

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Thanks! We'll be in touch soon.