Add support for snippetting.

[fulmicoton]

Tantivy offers a way to return a sample of the documents that contains some passage containiing the matched keywords.

It would be nice to add this feature to quickwit's API.

[evanxg852000]

I would go with :

• Rest: https://quickwit-host/api/v1/hdfs/search?query=body:foo&snippet_fields=body
• Cli: .quickwit index search --query "body:foo" --snippet_field body

The following checks need to happen during search:

• Does snippet_field appear in the query or part of the default search fields?
• Is snippet_field of field type that supports snippeting (text, indexed ..) ?
• Serialise the snippet