feat(config)!: Reworked how configuration is loaded

The environment variable `AUTOMATION_CONFIG` has been renamed to
`AUTOMATION__ENTRYPOINT` and can now also be set in `automation.toml` by
specifying:
```
automation = "<path>"
```

Directly accessing the environment variables in lua in no longer
possible. To pass in configuration or secrets you can now instead make
use of the `variables` and `secrets` modules.

To set values in these modules you can either specify them in
`automation.toml`:
```
[variables]
<name> = <value>

[secrets]
<name> = <value>
```

Or specify the environment variables `AUTOMATION__VARIABLES__<name>` and
`AUTOMATION__SECRETS__<name>` to set variables and secrets respectively.
By adding the suffix `__FILE` to the environment variable name the
contents of a file can be loaded into the variable or secret.

Note that variables and secrets are identical in functionality and the
name difference exists purely to make it clear that secret values are
meant to be kept secret.

Dockerfile

@@ -21,6 +21,6 @@ RUN cargo auditable build --release
 
 FROM gcr.io/distroless/cc-debian12:nonroot AS runtime
 COPY --from=builder /app/target/release/automation /app/automation
-ENV AUTOMATION_CONFIG=/app/config.lua
+ENV AUTOMATION__ENTRYPOINT=/app/config.lua
 COPY ./config.lua /app/config.lua
 CMD [ "/app/automation" ]