Support schema generation for `expand` option

[pawcoding]

Motivation

In our quest to add the expand parameter, generating the correct schema is the next logical step. This schema will then be used for the build time loader, and the resulting type can then also be used in the live loader.

Proposed Solution

Parse the expand option and request and process any additional PocketBase schemas needed. This includes:

• Generating the expand schema for every parameter specified
• Doing this recursively, up to 6 levels deep (limit by PocketBase)
• Check for single or multi relation fields

Additional Context

See #59 for more information