Add UI support for Standalone Multitenancy mode

[jsenko]

The new Standalone Multitenancy mode does not have any UI, since the standard UI is disabled. There are 3 ways of detecting the current tenant:

• Header
• Context path
• Subdomain

We can add UI support for the first two of these with minimal effort:

• For Header type, we can add a UI component to select the current tenant and use this value in API request headers. It could be as simple as a text box (no way of getting a list of tenants from the API).
• For Context path type, the UI servlet should serve the UI on the selected context path and pass that value to the UI. The UI could either only display the tenant ID, or provide a way of selecting a different one and redirecting.

As an alternative, we might reuse some of the control plane UI to wrap the data plane UI, but without any information that was provided by the control plane, so it would be still pretty bare. This UI would have to be served by the standalone Registry together with the current UI.

[apicurio-bot]

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