Add --sample flag to CLI invocation for run command

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

A flag --sample should be available when specifying dbt run.

Acceptance criteria

• Whether --sample is set is available in the ModelRunner
• Whether --sample is set is available in the JinjaContext

Suggested Tests

• Can get the value of --sample in the JinjaContext

Impact to Other Teams

N/A

Will backports be required?

No, it's a new feature

Context

N/A