Refactor API request options for non IAM users, enforce JSON response format for SPARQL

[cubeddu]

Issue #, if available: #229

Description of changes:

• Added options for non IAM calls, to fix issues with openCypher and SPARQL connections using proxy server.
• Enforce JSON Accept header for SPARQL.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (d15fca2) 10.01% compared to head (a4ce4e6) 10.01%.

Files	Patch %	Lines
...explorer/src/connector/openCypher/useOpenCypher.ts	0.00%	2 Missing ⚠️
...s/graph-explorer/src/connector/sparql/useSPARQL.ts	0.00%	2 Missing ⚠️
...graph-explorer/src/connector/gremlin/useGremlin.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   10.01%   10.01%   -0.01%     
==========================================
  Files         407      407              
  Lines       30089    30091       +2     
  Branches      655      655              
==========================================
  Hits         3014     3014              
- Misses      26740    26742       +2     
  Partials      335      335              

Flag	Coverage Δ
unittests	10.01% <0.00%> (-0.01%)	⬇️

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[michaelnchin]

LGTM, thanks @cubeddu !