Support JSON Schema conditional validation (if/then/else) for required fields #23

Today cozyvalues-gen can mark a field as required only unconditionally (via @field {type} name without brackets) or optional (@field {type} [name]). There is no way to express "field X is required only when field Y has value Z" in the generated values.schema.json, even though JSON Schema Draft 7 supports this via if/then/else or allOf + dependencies.

Use case. In chart packages/extra/etcd (cozystack/cozystack), the fields backup.destinationPath, backup.endpointURL, backup.s3AccessKey, and backup.s3SecretKey are required when backup.enabled: true and should be left empty otherwise. Render-time validation with the Helm required function is already in place, but the dashboard would give users earlier, field-level feedback if the generated schema could express the condition.

Proposed annotation (sketch, open to bikeshedding):

## @typedef {struct} Backup - ...
## @field {bool} [enabled] - ...
## @field {string} [destinationPath] - ...
## @requiredIf enabled=true destinationPath endpointURL s3AccessKey s3SecretKey

which would emit:

"backup": {
  "properties": {...},
  "if":   {"properties": {"enabled": {"const": true}}},
  "then": {"required": ["destinationPath","endpointURL","s3AccessKey","s3SecretKey"]}
}

Context: cozystack/cozystack#2428 (review thread cozystack/cozystack#2428 (comment)).