Add a constructor on FieldEntry struct from a FieldType

[fmassot]

Is your feature request related to a problem? Please describe.
For quickwit (but I imagine that it can be useful for others), we need to be able to instantiate FieldEntry from arbitrary FieldType.

Describe the solution you'd like

impl FieldEntry {
    /// Creates a new field entry given a name and a field type
    pub fn new(field_name: String, field_type: FieldType) -> FieldEntry {
        assert!(is_valid_field_name(&field_name));
        FieldEntry {
            name: field_name,
            field_type: field_type,
        }
    }