Support custom authentication credentials for SPARQL

[jaw111]

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Graph Notebook Version (and Graph Database and Version used if applicable)
Graph Notebook: 4.6.1
Graph Database: Any SPARQL endpoint requiring authentication

Is your feature request related to a problem? Please describe.
I have a SPARQL endpoint that requires all requests made with SPARQL Protocol to be authenticated using either Basic or Bearer scheme. Currently the Graph Notebook only supports IAM authentication for Neptune using AWS Signature Version 4 to sign the requests, but that is not going to work for SPARQL endpoints in general.

Describe the solution you'd like
Allow configuring authentication credentials to use for SPARQL requests to non-Neptune SPARQL endpoints.

Additional context
n/a