Support additional flags for cocoapods helpers

[AliSoftware]

This allows our validate_podspec and publish_pod helpers to support additional flags to forward to the pod commands they wrap:

validate_podspec

• Add support for --allow-warnings, --sources=…, --private, --include-podspecs=…, --external-podspecs=…
• The command now automatically adds --include-podspecs="*.podspec" to the call if not provided explicitly, in order to support repos that contains multiple co-dependant .podspec where one needs the other(s) to be able to validate(†).

publish_pod

• Add support for --allow-warnings and --synchronous

Why?

The main goal is to support repos containing multiple co-dependant podspecs. Examples include the Gravatar SDK having both Gravatar & GravatarUI, or WordPress-Aztec-iOS and WordPress-Editor-iOS.

In those cases:

• The validation of one pod might require to include the other pod too in order for the linting to not fail compilation (hence the need for --include-podspecs=…)
• The pod trunk push requires --synchronous otherwise the push of the first pod will succeed, but when trying to push the second pod which has the first one as a dependency, it won't be able to find the newly-pushed first pod it depends on, because it would not have propagated to the CocoaPods' CDNs yet

Testing

I've tested those changes by doing a dummy 1.0.1-alpha tag / version on the Gravatar-SDK-iOS repo after making it point to this branch of the CI_TOOLKIT plugin.

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.