Fix: add ruamel serializer for decimal.Decimal values

[georgesittas]

This PR was motivated by the following issue (connection is BigQuery):

(.venv) ➜  playground cat models/*
-- models/child.sql
MODEL (
  name `project.george.child`,
  kind FULL
);

SELECT
  col
FROM `project.george.parent`

-- models/parent.sql
MODEL (
  name `project.george.parent`,
  kind FULL
);

SELECT
  CAST(299.99 AS NUMERIC(10, 2)) AS col

After running sqlmesh plan successfully against this simple project, the following command fails:

(.venv) ➜  playground sqlmesh create_test 'project.george.child' --query 'project.george.parent' 'select * from project.george.parent limit 1'
packages/ruamel/yaml/representer.py", line 347, in represent_undefined
    raise RepresenterError(f'cannot represent an object: {data!s}')
ruamel.yaml.representer.RepresenterError: cannot represent an object: 299.990000000