make sure NewIndexConnection uses index.private_host if it's available

[austin-denoble]

Problem

There was a bug caught while testing BYOC. The sdk.NewIndexConnection function does not properly account for index.private_host if it exists, and only uses index.host to resolve the connection.

Solution

Update sdk.NewIndexConnection to use index.PrivateHost > index.Host when one is present.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Test data plane operations with a BYOC index.