Use App Service managed identity version 2019-08-01

[chlowell]

Documentation: https://docs.microsoft.com/en-us/azure/app-service/overview-managed-identity#obtain-tokens-for-azure-resources

Changes in the new version:

• MSI_ENDPOINT -> IDENTITY_ENDPOINT
• MSI_SECRET -> IDENTITY_HEADER
• "secret" header replaced by "X-IDENTITY-HEADER"
• "clientid" parameter -> "client_id"
• params for user-assigned identity object id ("principal_id") and resource ID ("mi_res_id")
• expires_on is epoch seconds

For example:

curl "$IDENTITY_ENDPOINT?resource=https://vault.azure.net&api-version=2019-08-01" -H "X-IDENTITY-HEADER: $IDENTITY_HEADER"

Response:

{
    "access_token": "***",
    "expires_on": "1597772135",
    "resource": "https://vault.azure.net",
    "token_type": "Bearer",
    "client_id": "9eba17f1-..."
}