Description: As a user, I would like to have the ability to provide a custom domain for the routes configured for serverless applications.

Acceptance Criteria:

• On all import forms should show hostname input field for the route section if the resource selected is knative service.
• if user-provided hostname then add required validation for it
• if user-provided valid hostname then on save, create the domainMapping resource associating it to knative service.
• While editing Knative service user should be able to add/update host name.

Additional Info:

SRVKS-648 (serverless v1.13) starts custom domain by using a ne resource DomainMapping.
We would like to configure the DomainMapping from Dev/Admin console.

Documentation is still in progress by SRVKS-681 but the usage is same with upstream so we can refer to upstream's doc https://github.com/knative/docs/pull/3017.