Check env vars SENTRY_RELEASE and GAE_DEPLOYMENT_ID when detecting release name, as done by SDKs.

Fixes GH-2050

If the global config file does not exist in the home directory, look in the following paths:
- Linux: $XDG_CONFIG_HOME/sentry/sentrycli.ini and $HOME/.config/sentry/sentrycli.ini
- Mac: $HOME/Library/Application Support/sentry/sentrycli.ini
- Windows: {FOLDERID_RoamingAppData}
This enables users to keep their home dir clean, adhering to standards such as XDG on linux.

Fixes GH-1521

Add CLI command send-metric for emitting metrics to Sentry.
Add new command-parser that uses clap's Derive API. Future commands should use this as the Derive API makes it:
-Easier to read, write, and modify arguments and commands.
-Easier to keep the argument declaration and reading of argument in sync.
-Easier to reuse shared arguments.

Fixes GH-2001

Add information to contributing docs about using Derive API for future commands.