Add support for listing all test tags

[bkhouri]

Description

Once swiftlang/swift-testing#591 is resolved, it would be ideal to have an option where swift test can provide the list of all tags available in a given package.

This relates to swiftlang/swift-testing#591

Expected behavior

swift test is augmented to provide a unique list of available tags, which, optionally, can be used to enable/skip execution.

Actual behavior

There is no feature to list tags.

Steps to reproduce

No response

Swift Package Manager version/commit hash

No response

Swift & OS version (output of swift --version && uname -a)

No response