Add configurations for null value handing in metrics

[QMalcolm]

A bug was raised in MetricFlow recently (dbt-labs/metricflow#764) outlining how null values are not handled properly in some cases. The proposed solution (dbt-labs/metricflow#759) requires changes to the MetricInputMeasure protocol used by MetricTypeParams protocol on Metric protocol. Specifically two new optional properties need to be added

• join_to_timespine which should have a boolean value
• fill_nulls_with which should optionally have a integer value

In the pydantic object implementations

• join_to_timespine should default to false if not specified during parsing / deserialization
• fill_nulls_with should default to None if not specified during parsing / deserialization