Add Value::coerce_str (#11885)

# Description
Following #11851, this PR adds one final conversion function for
`Value`. `Value::coerce_str` takes a `&Value` and converts it to a
`Cow<str>`, creating an owned `String` for types that needed converting.
Otherwise, it returns a borrowed `str` for `String` and `Binary`
`Value`s which avoids a clone/allocation. Where possible, `coerce_str`
and `coerce_into_string` should be used instead of `coerce_string`,
since `coerce_string` always allocates a new `String`.

crates/nu-command/src/stor/create.rs

@@ -89,7 +89,7 @@ fn process(
                     new_table_name
                 );
                 for (column_name, column_datatype) in record {
-                    match column_datatype.coerce_string()?.as_str() {
+                    match column_datatype.coerce_str()?.as_ref() {
                         "int" => {
                             create_stmt.push_str(&format!("{} INTEGER, ", column_name));
                         }