Add BlockType, BlockTypeClient

[mitchnielsen]

• Adds BlockType struct in internal/api
• Adds BlockTypeClient interface in internal/client
• Adds GetBySlug in internal/client

Closes https://github.com/PrefectHQ/terraform-provider-prefect/issues/171

[mitchnielsen]

Putting in draft state since this is still under development.

[mitchnielsen]

fire. out of curiosity, what did you have to change to fix that Client interface error?

Just a typo, I had written BlockType instead of BlockTypes