[ES|QL] Composer and Synth API for SET instruction (elastic#238896)

## Summary

- Adds support for `SET` header commands in `synth` and `composer`
modules of `@kbn/ast-esql` package.

- Allows to add `SET` instructions to the beginning of query:

```ts
synth.query `SET a = 123; FROM index`;
esql `SET a = 123; FROM index`;
```

- Adds `synth` and `Parser` methods to construct only single header
command:

```ts
// Parser:
Parser.parseHeaderCommand('SET a = 123');
// synth:
synth.header `SET a = ${value}`;
```

- Adds `.header` template tag to `ComposerQuery` for adding header
commands to the query:

```ts
const query = esql `FROM index`;

query
  .header `SET a = 123`
  .header `SET b = 456`;
```

- Adds `ComposerQuery` helper methods for working with header
*set*-instructions:
- `query.addSetCommand(name, value)` - add a `SET` instruction to the
query
- `query.removeSetCommand(name)` - remove specific header `SET` commands
  - `query.clearSetCommands()` - remove all header `SET` commands
- `query.getSetCommands()` - retrieve all `SET` instructions as a
lossless array
- `query.getSetCommnadRecord()` - retrieve flattened `SET` instruction
`Record`


### Checklist

- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

Co-authored-by: Stratou <[email protected]>

Author: 2 people

src/platform/packages/shared/kbn-esql-ast/src/ast/is.ts

@@ -24,6 +24,9 @@ export const isQuery = (node: unknown): node is types.ESQLAstQueryExpression =>
 export const isCommand = (node: unknown): node is types.ESQLCommand =>
   isProperNode(node) && node.type === 'command';
 
+export const isHeaderCommand = (node: unknown): node is types.ESQLAstHeaderCommand =>
+  isProperNode(node) && node.type === 'header-command';
+
 export const isFunctionExpression = (node: unknown): node is types.ESQLFunction =>
   isProperNode(node) && node.type === 'function';
 
@@ -72,7 +75,7 @@ export const isIntegerLiteral = (node: unknown): node is types.ESQLIntegerLitera
 export const isDoubleLiteral = (node: unknown): node is types.ESQLIntegerLiteral =>
   isLiteral(node) && node.literalType === 'double';
 
-export const isBooleanLiteral = (node: unknown): node is types.ESQLStringLiteral =>
+export const isBooleanLiteral = (node: unknown): node is types.ESQLBooleanLiteral =>
   isLiteral(node) && node.literalType === 'boolean';
 
 export const isTimeDurationLiteral = (node: unknown): node is types.ESQLTimeDurationLiteral =>

src/platform/packages/shared/kbn-esql-ast/src/composer/README.md

@@ -245,6 +245,39 @@ const query = esql.from('index').where`status == ${status}`
   .limit(50);
 ```
 
+#### SET Header Instructions
+
+ES|QL supports SET instructions at the beginning of queries to configure query-level settings. You can add SET instructions either directly in the template or programmatically:
+
+```ts
+// Direct in template (recommended)
+const query = esql`SET a = 123; FROM index | LIMIT 10`;
+
+// Multiple SET instructions
+const query = esql`
+  SET threshold = 100;
+  SET limit = 50;
+  FROM logs | WHERE value > ?threshold | LIMIT ?limit`;
+
+// Programmatically add SET instructions
+const query = esql`FROM index | LIMIT 10`;
+query.addSetCommand('setting1', 'value1');
+query.addSetCommand('setting2', 42);
+query.addSetCommand('setting3', true);
+
+// Get all SET instructions
+const sets = query.getSetCommands();
+// Returns: [{ name: 'setting1', value: '"value1"' }, ...]
+
+// Remove specific SET instruction
+query.removeSetCommand('setting2');
+
+// Clear all SET instructions
+query.clearSetCommands();
+```
+
+SET instructions support string, numeric, and boolean values and are preserved when combining queries or adding more commands.
+
 #### Output Methods
 
 ```ts