Add prefix_string config option for ML models (#2363)

The prefix_strings option was added to support the E5 model in
elastic/elasticsearch#102089

Author: davidkyle

output/openapi/elasticsearch-serverless-openapi.json

@@ -196,6 +196,7 @@ export class TrainedModelConfig {
   metadata?: TrainedModelConfigMetadata
   model_size_bytes?: ByteSize
   location?: TrainedModelLocation
+  prefix_strings?: TrainedModelPrefixStrings
 }
 
 export class TrainedModelConfigInput {
@@ -423,3 +424,14 @@ export class TrainedModelLocation {
 export class TrainedModelLocationIndex {
   name: IndexName
 }
+
+export class TrainedModelPrefixStrings {
+  /**
+   * String prepended to input at ingest
+   */
+  ingest: string
+  /**
+   * String prepended to input at search
+   */
+  search: string
+}