[action][cocoapods] remove all instances of is_string in options and use type

[crazymanish]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

• is_string is slowly being replaced by type to make the docs clearer, options safer, and Swift generation more correct.
• This PR does this for only cocoapods action.

Description

• Remove is_string: true from options
• Added type: Boolean

Testing Steps

• No functionality changed, all existing/new unit tests should pass.

[crazymanish]

This string_callback 🤯
Q: do I need to keep default_value: nil for this type? 🤔
(My logic is because this param has optional: true, so we don't need default_value: nil but I could be wrong too...)

[joshdholtz]

optional: true should be all that we need here 💪

[joshdholtz]

🔥

[fastlane-bot]

Hey @crazymanish 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀