EES-5974 infrastructure changes for search function app related to EES-5875

[benoutram]

This PR makes infrastructure changes for the Search Azure Function App to support changes made to it in EES-5875 by #5703. These changes are possible now that the configuration for the Azure AI Search resource was added by #5733.

• Rename the indexName app setting and parameter names to indexerName to match Azure's SearchIndexerClient class since the operation to retrieve indexer information requires the indexer name rather than the index name.
• Configure the app with the Azure AI search REST API endpoint.
• Configure the app with the Azure AI search indexer name.
• Enable the app's search indexer health check by uncommenting AzureSearchHealthCheckStrategy.
• Assign the app's system managed identity the 'Search Service Contributor' role required to list indexers.
• Break out a new searchServiceRoleAssignment template and refactor searchServiceConfig to use it.

Updated health check response after deploying these changes:

[leeoades]

Absolutely amazing stuff, Ben! Ship it!!