[#211][#207] initial demo setup with declarative approach and springboot approach + command dispatching/handling

[laura-devriendt-lemon]

Multi-tenancy example demos

Adds a set of runnable university examples under examples/university-multi-tenancy-examples
that demonstrate the Axoniq Framework 5 multi-tenancy support, so the feature has living
documentation and is exercised end to end.

Demonstrates #205 (tenant infrastructure), #207 and #211 (Spring Boot auto-configuration) at the moment.
Part of #176.

What the demo shows

• Per-tenant component injection: one CourseStatsStore and one AuditLog instance per
  tenant, with the framework injecting the right tenant's instance into both a command
  handler and a query handler.
• Tenant isolation: each tenant reads back only its own statistics.
• Adding a tenant at runtime and having its per-tenant components appear on first use.
• Rejecting a command dispatched for an unknown tenant.
• routing commands via the correct tenant context
• Closing a tenant's per-tenant instances when the tenant is removed and on shutdown.
• Filtering the Axon Server _admin context out of the discovered tenants.
• The configuration-time guardrail that rejects two providers for one component type.
• Disabling multi-tenancy through axon.multitenancy.enabled=false.

Modules

The demo is split so a reader can pick the module matching the configuration style they
want to learn.

Module	What it is
university-multi-tenancy-core	The demo itself: the university model and the tenant lifecycle, independent of how the application is configured. A library, not runnable on its own.
university-multi-tenancy-declarative	Runs the demo through the Configuration API. In memory by default, or against Axon Server via the demo.axon-server.enabled toggle.
university-multi-tenancy-springboot	Runs the demo through Spring Boot auto-configuration, against Axon Server (tenants are Axon Server contexts).

Tests

• MultiTenancyDemoTest (declarative): the tenant lifecycle end to end plus the
  provider-ambiguity guardrail.
• MultiTenancyDisabledTest (Spring Boot): with multi-tenancy disabled, dispatching a tenant-scoped command fails at runtime.
• MultiTenancyDemoIT (Spring Boot): boots Axon Server through Testcontainers and asserts
  the full lifecycle, including _admin filtering.

CI

The integration test needs a multi-context (Enterprise Edition) Axon Server, which requires
a license. A "Write Axon Server test license" step writes the AXON_SERVER_LICENSE secret
to the license file the test reads. The integration test runs in this repository's own pull
requests (where the secret is available) and locally (using a local license file), and skips
itself only where a license cannot be provided, such as fork pull requests. The license file
is git-ignored and never committed.

[hatzlj]

lgtm, just one cosmetic correction regarding axon server

[smcvb]

In all I love the structure an thoroughness. Definitely hope this helps people to comprehend how multi-tenancy works for Axon Framework. But, with 25 comments, I would've requested changes on this already merged PR.