Defines custom models for verticals

[aborem]

Closes #60

This PR defines the models for the different verticals we support in pardner at the moment. I opted to use Pydantic to define these models since it's flexible, leverages a lot of typing concepts already built into Python, and is easy to serialize/deserialize.

As a reminder, we don't have plans to store any data in pardner, that's the responsibility of the user of the library.

Next up:

• Replace references to Vertical enum in codebase with references to newly created base vertical class #61
• Return instance of BaseVertical model rather than Any for all Tumblr methods #62
• Return instance of BaseVertical model rather than Any for all GroupMe methods #63
• Return instance of BaseVertical model rather than Any for all Strava methods #64

[lisad]

Using pydantic here makes sense! I've seen talks about it at the local python user group so people I respect like it and I've been interested to see it used well in a project I am close to, I think this is it