[ES|QL] COMPLETION command grammar and logical plan (#126319)

Author: afoucret

docs/changelog/126319.yaml

@@ -0,0 +1,5 @@
+pr: 126319
+summary: COMPLETION command grammar and logical plan
+area: ES|QL
+type: feature
+issues: []

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.tokens

@@ -59,6 +59,7 @@ processingCommand
     | {this.isDevVersion()}? inlinestatsCommand
     | {this.isDevVersion()}? lookupCommand
     | {this.isDevVersion()}? changePointCommand
+    | {this.isDevVersion()}? completionCommand
     | {this.isDevVersion()}? insistCommand
     | {this.isDevVersion()}? forkCommand
     | {this.isDevVersion()}? rerankCommand
@@ -296,3 +297,7 @@ rrfCommand
 rerankCommand
     : DEV_RERANK queryText=constant ON fields WITH inferenceId=identifierOrParameter
     ;
+
+completionCommand
+    : DEV_COMPLETION prompt=primaryExpression WITH inferenceId=identifierOrParameter (AS targetField=qualifiedName)?
+    ;