feat: add provider specification

[smizell]

This adds a JSON Schema provider-spec.yaml for specifying provider definitions.

Decisions

• I went with JSON Schema because it was best for describing the JSON format we're using
• I tried to align the security schemes with OpenAPI 3.1
• I kept the use of id as a way to define identifiers for services and reused it for security schemes

[vercel[bot]]

@smizell is attempting to deploy a commit to the Superface Team on Vercel.

To accomplish this, @smizell needs to request access to the Team.

Afterwards, an owner of the Team is required to accept their membership request.

[smizell]

@zdne addressed comments.

[zdne]

Looks great!

Also see rendered https://supermodel.io/superface/ProviderDefinition?view=graph