docs(0135): align endpoints for a harmonized push and pull mechanism

[nicoprow]

This pull request aims to harmonize and optimize the Pull/Push use cases for the certificate management standard CX-0135.

We aim to change the API endpoints considerable to align more with the industry core standard for notification APIs for the push mechanism. At the same time we want to move away from certificates directly hosted as single Assets on the EDC. Instead, in this pull request we introduce endpoints to query and search company certificates directly at the API.

With pull and push both the consumer will now be only notified over the processing status of the certificate. The actual certificate will now need to be queried from endpoints which hold the certificate metada as well as file documents.

Contributes to https://github.com/catenax-eV/cx-ex-comp-cert-management/issues/23

With the introduction of a separate search endpoint for certificates also contributes to https://github.com/catenax-eV/cx-ex-comp-cert-management/issues/22

Certificates have a language code and can now contain multiple document files that can be acquired independently which contributes to https://github.com/catenax-eV/cx-ex-comp-cert-management/issues/25

Added a "UNDER_CERTIFICATION" status code to the request endpoint contributing to this issue: https://github.com/catenax-eV/cx-ex-comp-cert-management/issues/24

View the API specification

One way to render the OpenAPI specification showing the proposed REST API without any additional tools:

1. Go to: https://editor.swagger.io
2. Go to File -> Import URL
3. Copy paste https://raw.githubusercontent.com/nicoprow/catenax-ev.github.io/aef91b7cebbc64fcea8973cd66f154fa9c8c693f/docs/standards/CX-0135-CompanyCertificateManagement/assets/openapi-spec.yaml
4. Submit