Gate using sample mode with dbt_experimental_sample_mode environment variable

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

During the initial development of sample mode, when the details may change often, we want to gate even being able to use sample mode behind an environment variable dbt_experimental_sample_mode. We want to do this because we don't want anyone depending on sample mode until it is no longer an experimental feature (when we're confident it is no longer going to change dramatically / when 1.10.0 goes GA).

Acceptance criteria

There exists an environment variable which gates all sample mode functionality

Suggested Tests

• sample mode can be run when the environment variable is set to true
• sample mode cannot be run when the environment variable is set to false

Impact to Other Teams

N/A

Will backports be required?

No, it's a new feature

Context

N/A