Mix: Integrate cover traffic to mix service

[youngjoon-lee]

1. What does this PR implement?

This PR integrates the cover traffic module (implemented in #910) to the MixService.

Regarding CoverTrafficSettings, I've seen that some parameters in the CoverTrafficSettings can be derived from parameters in other settings (e.g. num_hops). So, I defined the CoverTrafficExtSettings that contains only parameters that the user must specify. All other parameters are derived from other settings.

2. Does the code have enough context to be clearly understood?

Yes

3. Who are the specification authors and who is accountable for this PR?

@youngjoon-lee @danielSanchezQ

4. Is the specification accurate and complete?

Yes

5. Does the implementation introduce changes in the specification?

No

Checklist

Warning

Do not merge the PR if any of the following is missing:

• 1. Description added.
• 2. Context and links to Specification document(s) added.
• 3. Main contact(s) (developers and specification authors) added
• 4. Implementation and Specification are 100% in sync including changes. This is critical.
• 5. Link PR to a specific milestone.