Mach-composer

This major releases introduces support for plugins in mach-composer. All previous
supported plugins (also called integrations) are still bundled with the
exectutable but will in the future be distributed separately.

With this release it is also possible to use build custom plugins. A plugin
should be created with the mach-composer-plugin-sdk and the resulting executable
should be available on your $PATH as `mach-composer-plugin-<plugin>`

- Interpolate variable references in the global config block

- Fix an encoding bug when passing sops encrypted variables to the generated
terraform file.

- Fix another config inheritance issue in the sentry plugin

- Fix serialization issue for variables when the key was not a string. We now
force keys of maps to be strings.
- Fix config inheritance issue in the sentry plugin (global -> component)

- Validate the AWS Account ID as a string, fixes an issue if the account starts
with 0.
- Set default sentry terraform provider version