Clarify default text options // add a string type

Need to check the following points:

• can we index with freq and positions in quickwit?
• what are the tokenizer available and how to add new ones?

Adding a "string" field type could be convenient for several reasons:

• very simple default configuration: want full text search => text field, if you only want to store some strings like email, id => string field
• Elastic use also two types, "text" and "keyword"

On cons is that the two data types store a string, so we're just adding a type to simplify configuration.

One more thing, if we want to add such a type, it's better to do it now.