Record specific measurements from the agent

The agent application should accept a custom command line argument to define custom measurements. For instance we could record the openssl version, or the kernel version, ...

We shouldn't be concerned about sensitive information since the db should not record these. Hence we can just pass the values to the command line even if they come from ENV.

A possible usage would be:

```
crank-agent --record "system/openssl=$(openssl version -v)"
```

- The name and value are separated by the first `=` char.
- Switches could be `-r|--record`.
- For each benchmark that this agent runs the value would be added automatically.
- The argument usage should be repeatable.