feat: add support for dgraph connection strings

[shivaji-kharse]

This PR adds support to open gRPC endpoints using "connection strings", specifically the dgraph.Open() function.

dgraph://{username:password@}host:port?args

username and password are optional. If username is provided, a password must also be present. If supplied, these credentials are used to log into the cluster thru the ACL mechanism.

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable

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[mangalaman93]

Do we have any existing test that use old way of connecting? If yes, we should replace those to use Open function. LGTM otherwise, approval should be from someone who understands JS more than me.

[matthewmcneely]

Looks good. Don't forget to update the README. You can look at the dgo README as an example.

[octref]

small nitpick but otherwise look good to me

[mangalaman93]

LGTM, differing to @octref for final approval. Thanks