Provide default mocks in table_meta

[Somtom]

Problem

At the moment we still need a lot of repetitive code to define our tests: Each from_mocks call needs to specify all the input table mocks even if their data would not change between tests.

What changed

We added a default_inputs argument to the table_meta decorator. This allows to pass reasonable default to the model.
Here is an example from the docs:

@table_meta(
    query_path="./examples/test_query.sql",
    default_inputs=[UserTable([]), SubscriptionTable([])] # We can provide defaults for the class if needed.
)
class MultipleSubscriptionUsersTable(BigQueryMockTable):
    user_id = col.Int(default=1)

# Setting up different scenarios to demonstrate the use of defaults
users = UserTable.from_dicts([{"user_id": 1}, {"user_id": 2}])
subscriptions = SubscriptionTable.from_dicts(
    [
        {"subscription_id": 1, "user_id": 1},
        {"subscription_id": 2, "user_id": 1},
        {"subscription_id": 2, "user_id": 2},
    ]
)

# Utilizing the default inputs set in the table_meta
res = MultipleSubscriptionUsersTable.from_mocks(input_data=[])
res = MultipleSubscriptionUsersTable.from_mocks(input_data=[users]) # Using only users, defaults for others
res = MultipleSubscriptionUsersTable.from_mocks(input_data=[users, subscriptions]) # Overriding defaults

What to look for

You can ignore the changes in docs/ since they are auto generated from the docsource/ markdown file changes.

Instead, have a look at:

1. What do you think about the "API" of providing defaults this way
2. Are the docs and examples understandable
3. General correctness

[karolinastawicka]

I love this solution! It saves a lot of time and allows you to only add/change the specific fields you want - especially useful if it's more than 3 column like in example.

Maybe worth adding in the default_values.md.txt that another plus of it is that it allows you to change only one column (or only those that you really need to change in your test case, not redefine all). If it is clear somewhere else in the documentation, and I didn't spot it, then ignore this comment.

🦐

[Somtom]

@karolinastawicka thanks for your review! I loved the suggestions and tried to add them with my changes.