[CT-2346] Cut over current semantic objects and helpers from MetricFlow

[QMalcolm]

This repository / package is intended to be where the semantic interface protocols live such that MetricFlow and dbt-core (and other projects) have shared importable understood protocol. That's a tall order. It all starts though with the current definitions as they are in MetricFlow, specifically everything that lives in the model director. The only carve outs from that are files & directories for data warehouse validations and dbt-metrics -> MetricFlow conversion. Data warehouse validations will remain part of MetricFlow, and conversions from dbt-metrics to MetricFlow will no longer be needed.

[QMalcolm]

This should also include the tests associated with these classes/functions

[QMalcolm]

Since originally creating this issue, we've moved in the direction of first separating out in MetricFlow what should be in dbt-semantic-interfaces. In that regard, the relevant code is currently being moved into metricflow/dbt_semantic_interfaces. Once validations, time, and tests have been moved into that sub directory, and once nothing in that sub directory refers to metricflow, then we can safely cut that directory to this repository.