Support external backends

[jesper-friis]

Description:

Add support for users to provide their own backends.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist for the reviewer:

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[codecov-commenter]

Codecov Report

Base: 67.52% // Head: 67.36% // Decreases project coverage by -0.17% ⚠️

Coverage data is based on head (7a20809) compared to base (68a30ed).
Patch coverage: 66.66% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   67.52%   67.36%   -0.17%     
==========================================
  Files          14       14              
  Lines        1124     1146      +22     
==========================================
+ Hits          759      772      +13     
- Misses        365      374       +9     

Impacted Files	Coverage Δ
tripper/triplestore.py	75.49% <65.38%> (-1.28%)	⬇️
tripper/__init__.py	100.00% <100.00%> (ø)

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