Index patterns - REST api for runtime field CRUD operations #92126
Description
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}