Builds on #72360

Similar to how we have REST API for managing deprecated scripted_fields we should add similar APIs for managing runtime fields:

• Runtime Fields API
  ** Create a new runtime field — POST /api/index_patterns/index_pattern/{id}/runtime_field
  ** Read a runtime field — GET /api/index_patterns/index_pattern/{id}/runtime_field/{name}
  ** Update a an existing runtime field — POST /api/index_patterns/index_pattern/{id}/runtime_field/{name}
  ** Upsert a runtime field — PUT /api/index_patterns/index_pattern/{id}/runtime_field
  ** Remove a runtime field — DELETE /api/index_patterns/index_pattern/{id}/runtime_field/{name}