feat: add rake task to generate schema without server

[matthv]

Definition of Done

• Add rails forest_admin:schema:generate rake task that generates .forestadmin-schema.json without starting the Rails server or sending the schema to the Forest Admin API

Usage

# Generate schema at the default path (.forestadmin-schema.json)
rails forest_admin:schema:generate

# Show stack trace on error
rails forest_admin:schema:generate debug=true

 ### CI example

```bash
rails forest_admin:schema:generate
git diff --exit-code .forestadmin-schema.json

[DayTF]

Can't we leave the default path of the agent setup? why define a new prop for this ?

[matthv]

Good point, I initially added this to allow generating the schema elsewhere and comparing it with the existing one, but a simple git diff after generation does the same job. I'll remove it.