[App Service] az webapp deploy: Add --pull-identity argument to enable MSI authentication for pull based deployments

[dannysongg]

Related command
az webapp deploy

Description
Adding --pull-identity argument to allow for MSI based authentication when performing a pull based deployment.

pullIdentity can have 3 states;

• null: MSI will not be used. This matches the existing behavior (SAS or public access)
• system: Authenticates using system assigned identity
• {client-Id}: Authenticates using provided user assigned identity

Testing Guide
az webapp deploy -g {resource-group-name] -n {app-name} --src-url {package-url} --pull-identity system --type zip: Deploys a zip file from {package-url} using the system assigned identity of the web app to authenticate against the storage account.
az webapp deploy -g {resource-group-name] -n {app-name} --src-url {package-url} --pull-identity {client-id} --type zip: Deploys a zip file from {package-url} using the user assigned identity with id {client-id} to authenticate against the storage account
az webapp deploy -g {resource-group-name] -n {app-name} --src-url {package-url} --type zip: Behavior is unchanged. Deployment will pull with anonymous access or SAS token

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[yonzhan]

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