Supporting dynamic downsampling for enrollments query

[mikewilli]

Downsampling is an useful tool for enrollments queries on large experiments, but the easiest way to do this right now involves overriding the entire enrollments query to add a WHERE sample_id <= {value} clause without changing the rest of the template.

We should look into adding another parameter to the _build_enrollments_query_normandy function so that we can more easily specify a sample rate for enrollment queries. This should allow us to add support in metric-hub and metric-config-parser for a sample_rate (or similar) parameter that could get injected into the query from jetstream.

[danielkberry]

Ideally, this would work for mobile experiments as well, though they definitely suffer from this issue less

[mikewilli]

Yea true, might as well add it as an option to all of the enrollments queries.