Allow specifying an Event Hub to use as the default EntityPath for namespace connection string

[oising]

Description

• Add new property to EventHub model: IsDefaultEntity

Only one EventHub can be specified as the default entity path. The entity path can still be overridden by settings in integrations.

• Add test to validate EntityPath is in connection string, and is well-formed
• Add test to validate setting multiple default entities is invalid
• Updated playground to remove settings usage (but left setting usage in other tests)
• Updated documentation in README.md for integrations to mention new functionality

Currently the design means validation is done late when the connection string is requested.

API review required.

Fixes #7093

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Is this introducing a breaking change?
      • Yes
        • Link to aspire-docs issue (please use this breaking-change template):
      • No
        • Link to aspire-docs issue (please use this doc-idea template):
  • No

[oising]

If we can agree on tihs, I'll send a PR for the same thing in Azure Service Bus for queue/topic in EntityPath, or I can put it in this one