Allow for specification of a "relative" time window for sample mode

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

When using sample mode, we need to allow for the specification of a "relative" time window, i.e. "the last three days". This might look something like dbt run --sample "3 days". The ending time of the sample would then be "the ending of today" and the beginning would be "the start of the day 2 days ago".

Acceptance criteria

• A relative time window can be specified as a CLI arg for sample mode invocations

Suggested Tests

N/A

Impact to Other Teams

N/A

Will backports be required?

No, it's a new feature

Context

N/A