Support encrypted backends for credentials

## Overview

The `~/.credentials` could be improved for supporting different back-end alternatives. This would be very useful for local testing where users will not need to store credentials in plain text on disk

## Requirements

- Support, at least, 1password as an alternative back-end for these credentials file.
- Make it easy to extend for other types of back-ends.
- Do not break current default behaviour. Make all this feature optional

## Approach
- Extend `compliance/utils/credentials.py` so that it supports multiple backends. The default behaviour should be the current local file.
- `1password` backend: users could use the Auditree configuration to specify the type of the backend they want to use. For example:

```json
"creds": {
"backend": "1password",
"domain": "my-domain"
}
```

In this case, Auditree will wait for an `auditree` vault in 1password (this vault name can be part of the config too) which will contain the credentials with the following schema:

- Name of sections of the credentials file will match with the name of a secure note.
- Each section key/value of the credentials file will be a field in the secure note.

For example: `creds['aws'].username` will be searched at `op://auditree/aws/username`

## Test Plan

No configuration provided should keep the same behaviour so far.