Add reconciliation API endpoint to REST API

[UnniKohonen]

This PR adds a Reconciliation Service API endpoint to REST API as mentioned in #338. It allows reconciling against a project using the /v1/projects/{project_id}/reconcile API method. It also includes a suggest service for entities with the /v1/projects/{project_id}/suggest/entity API method.

The main reconciliation endpoint accepts GET and POST requests:

• A GET request with no parameters returns a service manifest.
• AGET request with a URL query parameter queries serialized as JSON returns a reconciliation result batch.
• A POST request with an application/x-www-form-urlencoded parameter queries serialized as JSON returns a reconciliation result batch.

Only the fields query and limit are supported in the queries objects. Reconciliation result objects contain the fields id, name, score and match. Reconciliation doesn't support types or properties for now.

The reconciliation API specification requires the usage of the identifierSpace field in the service manifest. Since Annif doesn't require entities to be in a specific URI space and doesn't have a way to extract this information from a project, the field is left empty for now.

The suggest service endpoint accepts GET requests and returns a suggest response. It supports the prefix and cursor parameters.

Closes #338

[osma]

Excellent work!

The reconciliation API specification requires the usage of the identifierSpace field in the service manifest. Since Annif doesn't require entities to be in a specific URI space and doesn't have a way to extract this infomation from a project, the field is left empty for now.

I opened an issue about this problem: https://github.com/reconciliation-api/specs/issues/139

I will soon provide more detailed comments about the code.

[sonarcloud[bot]]

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