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_plugin_formats/src/from/ics.rs

@@ -9,7 +9,7 @@ pub const CMD_NAME: &str = "from ics";
 
 pub fn from_ics_call(call: &EvaluatedCall, input: &Value) -> Result<Value, LabeledError> {
     let span = input.span();
-    let input_string = input.coerce_string()?;
+    let input_string = input.coerce_str()?;
     let head = call.head;
 
     let input_string = input_string

crates/nu_plugin_formats/src/from/ini.rs

@@ -5,7 +5,7 @@ pub const CMD_NAME: &str = "from ini";
 
 pub fn from_ini_call(call: &EvaluatedCall, input: &Value) -> Result<Value, LabeledError> {
     let span = input.span();
-    let input_string = input.coerce_string()?;
+    let input_string = input.coerce_str()?;
     let head = call.head;
 
     let ini_config: Result<ini::Ini, ini::ParseError> = ini::Ini::load_from_str(&input_string);

crates/nu_plugin_formats/src/from/vcf.rs

@@ -8,7 +8,7 @@ pub const CMD_NAME: &str = "from vcf";
 
 pub fn from_vcf_call(call: &EvaluatedCall, input: &Value) -> Result<Value, LabeledError> {
     let span = input.span();
-    let input_string = input.coerce_string()?;
+    let input_string = input.coerce_str()?;
     let head = call.head;
 
     let input_string = input_string