feat: add get_dashbaord support for neptune

[Owen-CH-Leung]

Summary of Changes

Currently, using AWS Neptune as the backend, the get_dashboard function is not implemented at the NeptuneGremlinProxy, AbstractGremlinProxy and BaseProxy, which means that the dashboard at frontend does not display properly due to the missing implementation. For details, you can refer to this chat :

https://amundsenworkspace.slack.com/archives/C01A87A5EUU/p1653483057967299?thread_ts=1653374819.089879&cid=C01A87A5EUU

Hence, this PR attempts to create such functionality so that dashbaord works fine using Neptune as backend.

Tests

I tested the code using my company's aws environment (which uses AWS neptune & opensearch as the backend) and it worked fine.

Documentation

N/A

CheckList

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