Add config setting and default_engine_factory for visionOS.

[tfg4]

Add config setting and default_engine_factory for visionOS.

[linux-foundation-easycla]

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• ✅ login: tfg4 (287cf4b, 4cafb12, b8ad416, 72c5852)

[veblush]

@sampajano Would you look at this? If we want to add new visionos build configuration, do we need to do for other builds such as podspec?

[sampajano]

@veblush Thank you for forwarding it to me!!

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@HannahShiSFB Hi Hannah, could you help take a look at this request, and check if there's other places that we need to match it like mentioned above? Thanks!!

[sampajano]

@tfg4 Hi!

We have worked on another PR for adding visionOS support to grpc here:
#36476

That PR has all accompanying podspec change too, so i think we will use that one instead of this PR.

Thanks for your contribution!