infra(all): Introduce generic script for integration tests

[daymxn]

Per b/450976183,

This adds a generic command for SDK teams to decrypt their secret files for integration tests. Note that I also (locally) have subcommands for encrypt and run, but have not included those in this PR to keep the diff small.

More broadly, this PR introduces a new swift package called repo which is intended to be a point of migration for our larger bash scripts that are harder to maintain in bash form. Currently, it only includes the tests target for integration tests, but it's intended to be expanded upon as we deem necessary. The main point of this PR is to discuss whether we find this valuable or not. I felt it would be easier to integrate the generic commands for testing if I could take advantage of ArgumentParser, which led me to the idea of centralizing our larger bash scripts here. @ncooke3 greenlit the idea, but also felt it would be worth having further discussion in the PR.

As far as the specifics on decrypt, I have expanded it from #15398 to be more generic. You can now call it inline with generic parameters specifying the secret files and passphrase, or use a predefined JSON file to specify the secrets, and defer to an environment variable for the passphrase. You can also toggle the overwrite behavior (whether to leave existing decrypted files alone, or overwrite them).

Example usage of doing everything inline:

./scripts/repo tests decrypt --password "super_secret" \
  scripts/gha-encrypted/FirebaseAI/TestApp-GoogleService-Info.plist.gpg:FirebaseAI/Tests/TestApp/Resources/GoogleService-Info.plist \
  scripts/gha-encrypted/FirebaseAI/TestApp-GoogleService-Info-Spark.plist.gpg:FirebaseAI/Tests/TestApp/Resources/GoogleService-Info-Spark.plist

Example usage of using a JSON file and an environment variable for the passphrase:

./scripts/repo tests decrypt --json ./scripts/secrets/AI.json

#no-changelog

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[ncooke3]

I like this. It does add a lot more code than the current decryption approach, but it's easier to fine tune in Swift than bash (IMO) and this will hopefully scale better with additional subcommands (build, etc.).