Support custom authentication credentials for SPARQL

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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](https://datatracker.ietf.org/doc/html/rfc7617) or [Bearer](https://datatracker.ietf.org/doc/html/rfc6750) 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