Fixes remote attachments testing in example app

[cameronvoell]

Revives and exposes commands for remote attachment testing from previous work #85

See new README instructions:

If testing the remote attachment content type, run (from example directory):

yarn run upload:up

If running on andoid also run:

adb reverse tcp:8443 tcp:8443

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[nplasterer]

What do you think about bringing this test back from the legacy branch as well?

https://github.com/xmtp/xmtp-react-native/blob/xmtp-legacy/example/src/tests/tests.ts#L686-L773

I think a few tests from that file are probably still relevant even though I removed the whole file in the upgrade.