[e2e] Creates basic support/documentation/example to iOS.

[AlbertWang0116]

Description

* Updates iOS E2EPlugin and add skeleton iOS test case E2EIosTest.
* Adds instructions to README.md about e2e testing on iOS device.
* Applies iOS e2e to example.

Related Issues

Checklist

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• [x ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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• I updated/added relevant documentation (doc comments with ///).
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• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[AlbertWang0116]

FYI the context behind this:

If the app statically links to plugins, the test target also needs to statically link to the same library. This is a known issue with CocoaPods. If we want to keep the static linking, we need to tell Podfile not to inherit search path from the main target[1], and remove $(inherited) in .xcconfig of the test target[2]. This is a tedious work for users, so I just put a simpler workaround (dynamic linking) here.

We can still allow the static linking, by resolving the additional steps mentioned above by flutter tools.

[AlbertWang0116]

[1] https://stackoverflow.com/a/45457045
[2] https://stackoverflow.com/a/46933115

[AlbertWang0116]

@collinjackson

[collinjackson]

Looks good, I have just a few minor suggestions.

[AlbertWang0116]

I made E2EIosTest fallback not to be a XCTestCase class for now. Otherwise iOS app won't run until developer manually open .xcodeproj and run a test case. Flutter tool should be able to resolve this, then we can switch E2EIosTest to XCTestCase.

[collinjackson]

Landing this as a functionality update since it introduces new API surface area on iOS.

As a follow-on PR, could you please update the documentation to explain how to run the tests on the command line (so that Firebase Test Lab can be used on CI)

[collinjackson]

Suggested change

	## 0.2.2+4
	## 0.2.3