[Core] Add ability to load version from product SDK

[ncooke3]

This PR adds support for SDKs outside of the firebase-ios-sdk repo to register with core. Like how the Swift SDKs inside of the firebase-ios-sdk repo are registered, it relies on hardcoding information about the SDK in core.

To support SDKs that may have a version different from core, they have the option of implementing the func sdkVersion() -> String method.

For example, the below code belongs to the Foo package that lives outside of the firebease-ios-sdk repo. If the below code is included in the Foo SDK, and the client depends on both Foo and the firebase-ios-sdk packages, Foo will register itself with core.

// Foo.swift
// Inside of the `Foo` package

@objc(FIRFooComponent) class FirebaseFooComponent: NSObject {
  @objc static func sdkVersion() -> String {
    return "12.0.0" 
  }
}

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[paulb777]

Should be good to merge after functionality confirmed and tested in firebase/data-connect-ios-sdk#8