story: Platform Mesh Authentication

[Perseus985]

User Story

As a Platform Mesh operator,
I want to make the Core installation multi-org capable,
so that multiple organizations can be supported even with a smaller footprint.

Acceptance Criteria

• An external Keycloak can be connected (with no loss of features).
• Alternatively, I can use a Keycloak provided by Platform Mesh itself.
• Multi-org capability is available with both variants (external and Platform Mesh-provided Keycloak).
• Multiple organizations can be created and managed within a single Core installation.
• Both variants are documented

Parent Epic

#290

Additional Context

For use in the Opendefense Cloud, infrastructure will exist in several sizes (data centers, deployable/field, mobile).
We want to manage and operate all stacks — across their different sizes — in a uniform way.
For exchange with other organizations, we need a multi-org capability in addition to the Core edition (#304).