Support multiple source roots

[lcian]

📜 Description

Adds support for multiple source roots.

Works similarly to our sentry-android-gradle-plugin:

• The contents of the source roots (+ any additional dirs passed with the additionalSourceDirsForSourceContext plugin parameter get copied to target/sentry/collected-sources
• Then they are bundled and sent by sentry-cli as normal

💡 Motivation and Context

Closes #70

💚 How did you test it?

Tested manually with the sample project

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

Document it

[lcian]

Working on checking that the existing integration tests still pass and adding new tests.

[lcian]

This PR should come after #142

[lbloder]

NIce work!
Added a comment regarding try-with-resource for the stream that is returned by Files.walk

[lbloder]

Nice, LGTM 👍