feat: query aggregate instances by id

[nanderstabel]

Description of change

Adds the endpoints mentioned in #129

Blocked by

• feat: add support for linked verifiable presentations #92

Links to any relevant issues

fixes issue #129

How the change has been tested

Tested through the updated Postman collection:

• Issuance/offer
• Issuance/credential
• Verification/authorization_request
• Holder/offer
• Holder/credential
• Holder/presentation

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have successfully tested this change in a docker environment

[daniel-mader]

I suggest renaming the commit message to something more precise:

feat: add individual aggregate instance endpoints
-->
feat: query aggregate instances by id