Make task rate limit a configuration option

[king-alexander]

💡 Summary

The Certificate Search API offers 3 pricing tiers: small, medium, and large. The task rate limit should be configurable to our level of access.

Motivation and context

This would be useful because we want the ability to process our full list of domains in a week or less.

Acceptance criteria

• [ ] I can configure admiral to work on each pricing tier with the appropriate rate limit in place