[Feature] Return agate_table in dbt run-operation result

[acjh]

Resolves #10956

Problem

dbtRunner does not propagate the return value of dbt run-operation.

Solution

Propagate the return value of dbt run-operation in the result as agate_table:

• In the run method of RunOperationTask, capture the return value of _run_unsafe() in a variable execute_macro_result and pass it when instantiating RunResult with agate_table=execute_macro_result.
• Define agate_table field in RunResultOutput, and propagate agate_result from RunResult to RunResultOutput in the class method RunResultsArtifact.from_execution_results.

Sample usage:

>>> from dbt.cli.main import dbtRunner
>>> dbt = dbtRunner()
>>> res = dbt.invoke(['run-operation', 'select_something_with_return', '--args', 'name: world'])
>>> res.result.results[0].agate_table.print_json()
[{"name": "hello, world"}]

Also:

• Added tests/functional/adapter/dbt_run_operations/test_dbt_run_operations.py.
• Delegated process_run_result to a class method of RunResultsArtifact to allow overrides if desired.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.

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[acjh]

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[acjh]

Note: When there is no explicit return function call in the macro — like for the select_something test — res is a str of newline characters and spaces, at least as observed with the Postgres and SQLite adapters.

[acjh]

A more robust check is not isinstance(res, agate.Table) but there appears to be deliberate avoidance of import agate. Let me know if that is preferred.