Support multiple domains via ConfigureCustomDomain

[matthebrown]

Description

Adds support for multiple domains via ConfigureCustomDomain by appending the domain to the domain list instead of replacing the list entirely.

Fixes #7143

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

[davidfowl]

It feels strange to make Configure* Add maybe it should be AddCustomDomain

[matthebrown]

It feels strange to make Configure* Add maybe it should be AddCustomDomain

AddCustomDomain feels more intuitive that calls can be chained to add multiple domains if desired.

[davidfowl]

We can't rename the method, we need to add an overload.