Add OCM Test Suite matrix workflow for Nextcloud

[MahdiBaghbani]

This PR is a part of cs3org/OCM-STA#3
This PR is a part of cs3org/OCM-STA#4

The document for this workflow can be found on OCM Test Suite

A demo of the run can be found in the GitHub Actions, you can also download and review the videos generated by Cypress from artifact section.

This pull request adds a dedicated GitHub Actions workflow that runs the OCM Test Suite against the Nextcloud server using a small, fixed share-with matrix.

• The workflow builds a four entry matrix:
  • Stub v1 <-> Stub v1
  • Stub v1 <-> Nextcloud (current branch)
  • Nextcloud (current branch) <-> Stub v1
  • Nextcloud (current branch) <-> Nextcloud (current branch)
• Each matrix entry runs the OCM Test Suite via the dev-stock image with scenario share-with, using the current branch code for any Nextcloud side of the test.
• The job uses a single matrix actor job and a simple images field to pull all required Docker images per combination.
• A summary job collects per combination status JSON artifacts, renders a small Markdown table into the GitHub Actions summary, and cleans up status artifacts while keeping Cypress videos and screenshots.
• The workflow is configured as non blocking: it uses continue on error and always runs the summary, so OCM regressions are visible but do not fail the entire CI run.

This gives Nextcloud a stable, easy to read OCM regression check on every change without changing any server side APIs.

The OCM-Stub is subject to be developed in the next milestone, and this workflow would also be updated, as of now it is expected that the results for the NC to Stub and Stub to NC to be failure.