Support authentication when load external definitions, especially loading external auth definitions

[Flex-Xuan]

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What would you like to be added: Support authentication when loading external definitions

Why is this needed: Since security is critical, so if I defined the AuthenticationDefinitions in an external places, when loading it, the external resource must do some authentication works (You don't want everyone can access your username/password, right?) currently, I haven't seen related codes to this in method LoadExternalDefinitionCollectionAsync