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-protocol/src/config/helper.rs

@@ -15,7 +15,7 @@ pub(super) fn process_string_enum<T, E>(
     E: Display,
 {
     let span = value.span();
-    if let Ok(v) = value.coerce_string() {
+    if let Ok(v) = value.coerce_str() {
         match v.parse() {
             Ok(format) => {
                 *config_point = format;

crates/nu-protocol/src/config/mod.rs

@@ -539,7 +539,7 @@ impl Value {
                                     process_bool_config(value, &mut errors, &mut config.filesize_metric);
                                 }
                                 "format" => {
-                                    if let Ok(v) = value.coerce_string() {
+                                    if let Ok(v) = value.coerce_str() {
                                         config.filesize_format = v.to_lowercase();
                                     } else {
                                         report_invalid_value("should be a string", span, &mut errors);

crates/nu-protocol/src/config/table.rs

@@ -272,7 +272,7 @@ pub(super) fn try_parse_trim_strategy(
 }
 
 fn try_parse_trim_methodology(value: &Value) -> Option<TrimStrategy> {
-    if let Ok(value) = value.coerce_string() {
+    if let Ok(value) = value.coerce_str() {
         match value.to_lowercase().as_str() {
         "wrapping" => {
             return Some(TrimStrategy::Wrap {