feat: expose field type and options in advanced settings API

[SantiagoSuHe]

Description

The Advanced Settings API (/api/contentstore/v0/advanced_settings/{course_id})
returns, for each setting, only value, display_name, help, deprecated,
and hide_on_enabled_publisher. It does not tell clients what type a
setting is, nor what valid choices an enum-like setting accepts.

This forces any UI that wants to render type-specific controls (toggles, number
fields, dropdowns) to hardcode that metadata on the client. That duplication
drifts from the platform over time and can cause silent data loss — e.g. a
dropdown that omits a value the course actually has set.

This PR moves that metadata to the backend, which already knows it, so it can be
the single source of truth.

Changes

• CourseMetadata.fetch_all now includes two additional fields per setting:
  • type — the XBlock field class name (String, Boolean, Integer,
    Float, List, Dict, …), so clients can choose an input type without
    inferring it from the value's shape.
  • options — the field's values: a list of {display_name, value}
    choices for enum-like fields, a constraints dict (e.g. {"min": 0}) for some
    numeric fields, or null for free-form fields.
• Adds course-level values to enum settings that lacked them, so their valid
  choices are defined in the backend:
  • showanswer, rerandomize, show_correctness (xmodule/modulestore/inheritance.py)
  • certificates_display_behavior (xmodule/course_block.py)
  • These reference a new xmodule/course_settings_field_options.py that holds
    the canonical option lists.
• Settings that already declared values (catalog_visibility,
  course_visibility, video_sharing_options) are exposed automatically by the
  serializer change — no per-field edit needed.

The problem-level field definitions in xmodule/capa_block.py are unchanged.
As a follow-up, those could be migrated to reference the same shared option lists
so the problem and course levels share a single source.

The response is additive and backward compatible: existing fields are untouched,
the two new keys are simply added.

Context

This is the backend counterpart to the Advanced Settings redesign frontend PR
openedx/frontend-app-authoring#3019. It addresses review feedback there that the
redesign should not hardcode field metadata in the frontend, but source it from
the backend instead.

Testing

• Verified CourseMetadata.fetch_all returns type and options for all
  settings via the CMS shell (all 8 enum settings now carry their options;
  booleans report type: "Boolean"; numeric fields report type: "Integer"/
  "Float").
• Verified end-to-end against the redesigned authoring MFE: enum settings render
  as dropdowns populated from the backend options, booleans as toggles, etc.

Draft

Opened as a draft as the companion to the frontend PR and to gather feedback on
the approach before requesting formal review.

🤖 Generated with Claude Code

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