Enable debug mode in `with_sentry_client`

[loewenheim]

There is a comment in with_sentry_client about debug mode: https://github.com/getsentry/sentry-cli/blob/554f4fcdcba74c203b628dd695ddbc89702db8e7/src/utils/event.rs#L84-L85

Since the help message of send-envelope (which uses with_sentry_client) says

Check debug output for transmission errors by passing --log-level=debug or setting SENTRY_LOG_LEVEL=debug.

we should probably go through with this.

[elramen]

This applies to send-event also

[elramen]

Instead of using with_sentry_client, we could probably use EnvelopesApi which will be added to sentry-cli