[Feature] Add spec changes for conversion metrics

[WilliamDee]

Is this your first time submitting a feature request?

• [X] I have read the expectations for open source contributors
• [X] I have searched the existing issues, and I could not find an existing issue for this feature
• [X] I am requesting a straightforward change to existing dbt-semantic-interfaces functionality, rather than a Big Idea better suited to a discussion

Describe the feature

The following will encompass the spec changes for conversion metrics.

metric:
  ...
  type_params:
    conversion_type_params:
      entity: _entity_ # required
      calculation: _calculation_type_ # optional. default: conversion_rate. options: conversions(buys) or conversion_rate (buys/visits) + more to come
      base_measure: _measure_ # required
      conversion_measure: _measure_ # required
      window: _time_window_ # optional. default: inf. window to join the two events on. Follows similar format as time windows elsewhere (i.e. 7 days)
      constant_properties: # optional. List of constant properties default: None
        - base_property: _string_ # required. A reference to a dimension/entity of the semantic model linked to the base_measure
          conversion_property: _string_ # same as base above, but to the semantic model of the conversion_measure

Describe alternatives you've considered

No response

Who will this benefit?

Enables the SL to build conversion metrics

Are you interested in contributing this feature?

No response

Anything else?

No response

[Jstein77]

@QMalcolm taking our new process for a spin. Will and I confirmed this spec change, and it's ready to go into DSI. I added a Spec Change Approved label to verify that a MF PM has validated the spec changes.

[QMalcolm]

Added via #209