Fix: disable endpoint discovery when a custom endpoint is set

[kevinwcyu]

Fixes: #452

In v1 of aws-sdk-go, endpoint discovery is disabled if Endpoint is set (see the v1 docs and search for EnableEndpointDiscovery to see the note about this).

This was changed in v2 of aws-sdk-go to auto, which enables it by default for timestream since it's required as noted here.

Timestream uses the DescribeEndpoints API call to execute this endpoint discovery. It does by calling an endpoint with the following base URL https://query.timestream.us-east-2.amazonaws.com/. If a user is running this from a VPC without allowing outbound connections to it, then the call will fail.

This PR handles this by disabling endpoint discovery only when an endpoint is explicitly set. This brings back the same behaviour that we had in v1 that allowed users to use a VPC configured timestream without needing to allow outbound traffic to https://query.timestream.us-east-2.amazonaws.com/ over the public internet.

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• yarn.lock change happened when i ran yarn && yarn build
• changes to pkg/main.go and to the files in .vscode were to make it possible to debug the backend changes. the nested .vscode/.vscode directory contained outdated configuration so i removed it.