azurerm_cdn_frontdoor_origin.private_link: add web_secondary and Gateway to target_type accepted values - #29380

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Description

Support web_secondary and Gateway as a private link target types. These are used for connecting to Azure Storage Static Web Sites and Azure API Management, respectively.
Also, fixed the documentation to add the supported backend services related to the supported target types. Some of them were already supported on the target type list, but they were not listed as supported services, like Azure Storage (Static Web Sites) and Azure Container Apps (preview), for instance.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• I have updated/added Documentation as required written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work.
  For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

Changes to existing Resource / Data Source

• I have added an explanation of what my changes do and why I'd like you to include them (This may be covered by linking to an issue above, but may benefit from additional explanation).
• I have written new tests for my resource or datasource changes & updated any relevant documentation.
• I have successfully run tests with my changes locally. If not, please provide details on testing challenges that prevented you running the tests.
• (For changes that include a state migration only). I have manually tested the migration path between relevant versions of the provider.

Testing

Just a validation change, test not required

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_cdn_frontdoor_origin.private_link - support for web_secondary and Gateway as values for the target_type property [cdn_frontdoor_origin.private_link.target_type doesn't accept value web_secondary #23113]

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #23113
Similar solution used to close #28115