Add support for naming zoekt AST filters for easier debugging

We would like to add support for naming filter components so they are easier to debug.

For example:

{
	"version": 2,
	"timeout": "120s",
	"num_context_lines": 1,
	"max_file_match_window": 1000,
	"max_file_match_results": 5,
	"max_line_match_window": 500,
	"max_line_match_results": 5000,
	"max_line_match_results_per_file": 3,
	"forward_to": [
		{
			"query": {
				"and": {
					"children": [
						{
							"_context": { // <---- Add this object
								"name": "search-for-test"
							},              
							"query_string": {
								"query": "test"
							}
						}
          ]
        }
			},
			"endpoint": "http://127.0.0.1:6070"
		}
	]
}