[Adaptive Job] Add freshness spec to model

[ChenyuLInx]

Housekeeping

• I am a maintainer of dbt-core

Short description

Introduce new configuration on the model to support adaptive job

models:
  - name: stg_orders
    freshness:
      # build this model after X amount of time, as long as it has new data
      build_after: 
        count: 4
        period: hour
        depends_on: all

Acceptance criteria

The new configuration will be parsed and represented on the model node after the parsing phase.

Suggested Tests

1. Test valid freshness config, parsing works
2. Test invalid freshnees config, user should get a clear parsing error.

Impact to Other Teams

This involve adding new fields to model node, so it will impact metadata team when it is done.

Will backports be required?

No

Context

No response