Fix: Enable neptune proxy custom sts endpoint

Summary of Changes

Currently, when users deploy Amundsen with Neptune backend DB, NeptuneBulkLoaderApi is instantiated with the default STS endpoint.

However, this might not work when users deploy the service in government cloud / regional cloud, and the default endpoint is disabled. This PR allows user to specify the STS endpoint in the metadata config file, if they wish to.

Tests

No tests added because it does not change any application behavior. It only creates an option for user to specify their STS endpoint upfront in the config file.

Documentation

Added a line in docs/tutorials/how-to-use-amundsen-with-aws-neptune.md to explain that users can now specify STS endpoint.

CheckList

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[x] PR adds unit tests, updates existing unit tests, OR documents why no test additions or modifications are needed.
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amundsen-io / amundsen