Add support for DeveloperPortal and OpenApiJson format

[solankisamir]

Description

• Added support for retrieving and configuring Custom Domain on the DeveloperPortal Endpoint
  ApiManagement New Developer portal custom domain is not returned in Az cmdlet #11007

• Export-AzApiManagementApi Added support for downloading Api Definition in Json format
  Export-AzApiManagementApi openapi+json #9987

• Import-AzApiManagementApi Added support for importing OpenApi 3.0 definition from Json document

• New-AzApiManagementIdentityProvider and Set-AzApiManagementIdentityProvider Added support for configuring Signin Tenant for AAD B2C Provider
  Allow to set SigninTenant for New-AzApiManagementIdentityProvider operation #9784

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here

[wyunchi-ms]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[solankisamir]

@wyunchi-ms can you take a look at this again