refactor: Clean up SPI

[bscholtes1A]

What this PR changes/adds

Move *-spi directory out of extensions.
Also relocate some stuff of resources into system-tests as it is done in https://github.com/eclipse-dataspaceconnector/IdentityHub/tree/main/system-tests.

Why it does that

Clean up.

Further notes

Linked Issue(s)

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• added relevant details to the changelog? (skip with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Base: 16.82% // Head: 16.82% // No change to project coverage 👍

Coverage data is based on head (0756242) compared to base (709b950).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #44   +/-   ##
=========================================
  Coverage     16.82%   16.82%           
  Complexity       37       37           
=========================================
  Files            32       32           
  Lines           523      523           
  Branches         26       26           
=========================================
  Hits             88       88           
  Misses          435      435           

Impacted Files	Coverage Δ
...ctor/registration/DataspaceRegistrationPolicy.java	0.00% <ø> (ø)
...ctor/registration/authority/model/Participant.java	0.00% <ø> (ø)
...egistration/authority/model/ParticipantStatus.java	0.00% <ø> (ø)

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