Update api-security-schemes lab with RBAC and fixtures

[StarKhan6368]

Summary

Updates the api-security-schemes lab to cover RBAC with OAuth using Specmatic before fixtures

Changes

• Explained how OAuth tokens are fetched using before fixtures instead of being configured as a static token in specmatic.yaml
• Added guidance for wiring the captured ACCESS_TOKEN into the Authorization header in OAuth examples.
• Updated the docker command to apply the required fixes across the lab files

Depends On

1. Sample: Add OAuth RBAC scopes and fixture examples specmatic-order-api-java-with-oauth#113
2. Central Repository: Add RBAC with OAuth to order-api-with-auth labs-contracts#5
3. Specademy: https://github.com/specmatic/specademy/pull/9

[StarKhan6368]

Before merging, we need to ensure that the Docker build has been published from the sample, the contract repository has been merged, and the branch is removed from the Specmatic config of the lab.