Enha: support uploading multiple source map files simultaneously

[mikkopiu]

Closes #141

Thanks for the plugin! I noticed the same issue as in #141 and would like to drop our custom shell script step, so here's a quick enhancement for that. I'm a Ruby-noob so let me know if something here should be done differently.

Some cases, such as when using Hermes builds with React Native on Android, require uploading both the bundle file and its corresponding source map simultaneously to be correctly processed by Sentry.

To avoid breaking backwards compatbility, use the same singular argument name but accept both string and array values.

[brustolin]

@denrase since you're our fastlane specialist, can you take a look at this?

[mikkopiu]

Thanks for the review, I'll check these next week!

[mikkopiu]

Simplified implementation as per review, fixed style issues, added one more tests to also cover the comma-separated list case and updated changelog. LMK if there's something else to fix!