Add completion implementation.

Author: afoucret

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.g4

@@ -84,10 +84,11 @@ WHERE : 'where'               -> pushMode(EXPRESSION_MODE);
 // Once the command has been stabilized, remove the DEV_ prefix and the {}? conditional and move the command to the
 // main section while preserving alphabetical order:
 // MYCOMMAND : 'mycommand' -> ...
+DEV_COMPLETION : {this.isDevVersion()}? 'completion'     -> pushMode(EXPRESSION_MODE);
 DEV_INLINESTATS : {this.isDevVersion()}? 'inlinestats'   -> pushMode(EXPRESSION_MODE);
-DEV_LOOKUP :      {this.isDevVersion()}? 'lookup_🐔'      -> pushMode(LOOKUP_MODE);
+DEV_LOOKUP :      {this.isDevVersion()}? 'lookup_🐔'     -> pushMode(LOOKUP_MODE);
 DEV_METRICS :     {this.isDevVersion()}? 'metrics'       -> pushMode(METRICS_MODE);
-DEV_RERANK :      {this.isDevVersion()}? 'rerank'       -> pushMode(EXPRESSION_MODE);
+DEV_RERANK :      {this.isDevVersion()}? 'rerank'        -> pushMode(EXPRESSION_MODE);
 // list of all JOIN commands
 DEV_JOIN :        {this.isDevVersion()}? 'join'          -> pushMode(JOIN_MODE);
 DEV_JOIN_FULL :   {this.isDevVersion()}? 'full'          -> pushMode(JOIN_MODE);
@@ -179,6 +180,7 @@ DECIMAL_LITERAL
 BY : 'by';
 ON: 'on';
 WITH: 'with';
+AS: 'as';
 
 AND : 'and';
 ASC : 'asc';
@@ -355,7 +357,7 @@ RENAME_DOT: DOT -> type(DOT);
 RENAME_PARAM : {this.isDevVersion()}? PARAM -> type(PARAM);
 RENAME_NAMED_OR_POSITIONAL_PARAM : {this.isDevVersion()}? NAMED_OR_POSITIONAL_PARAM -> type(NAMED_OR_POSITIONAL_PARAM);
 
-AS : 'as';
+RENAME_AS : AS -> type(AS);
 
 RENAME_ID_PATTERN
     : ID_PATTERN -> type(ID_PATTERN)

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

@@ -52,6 +52,7 @@ processingCommand
     | enrichCommand
     | mvExpandCommand
     // in development
+    | {this.isDevVersion()}? completionCommand
     | {this.isDevVersion()}? inlinestatsCommand
     | {this.isDevVersion()}? lookupCommand
     | {this.isDevVersion()}? joinCommand
@@ -330,9 +331,12 @@ inlinestatsCommand
     ;
 
 rerankCommand
-    : DEV_RERANK queryText=stringOrParameter ON input=primaryExpression WITH inferenceId=stringOrParameter
+    : DEV_RERANK queryText=stringOrParameter ON fields WITH inferenceId=stringOrParameter
     ;
 
+completionCommand
+    : DEV_COMPLETION prompt=primaryExpression WITH inferenceId=stringOrParameter (AS target=qualifiedName)?
+    ;
 
 joinCommand
     : type=(DEV_JOIN_LOOKUP | DEV_JOIN_LEFT | DEV_JOIN_RIGHT)? DEV_JOIN joinTarget joinCondition