Add GetHttpsCallableFromURL to Functions

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Adds GetHttpsCallableFromURL to the Functions SDK. This was recently added to both the Android and iOS SDKs.

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Testing

Describe how you've tested these changes.

Running the integration testapp locally
https://github.com/firebase/firebase-unity-sdk/actions/runs/2379519242

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Will require firebase/firebase-cpp-sdk#964, and updating the firebase_cpp default

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit c93a6a0
Last updated: Wed May 25 15:08 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [2019] [macos] [1/5 Platform(s): Android] [android_target]
functions	[BUILD] [ERROR] [2019] [macos] [All 3 Platform(s)] [TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Playmode]
messaging	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Android] [android_target]
storage	[BUILD] [ERROR] [2019] [macos] [All 3 Platform(s)] [TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Playmode]