Refactor config updates (#13802)

# Description
This PR standardizes updates to the config through a new
`UpdateFromValue` trait. For now, this trait is private in case we need
to make changes to it.

Note that this PR adds some additional `ShellError` cases to create
standard error messages for config errors. A follow-up PR will move
usages of the old error cases to these new ones. This PR also uses
`Type::custom` in lots of places (e.g., for string enums). Not sure if
this is something we want to encourage.

# User-Facing Changes
Should be none.

crates/nu-command/src/network/url/join.rs

@@ -300,14 +300,14 @@ impl UrlComponents {
             return Ok(true);
         }
         match key {
-            "host" => Err(ShellError::UnsupportedConfigValue {
-                expected: "non-empty string".into(),
-                value: "empty string".into(),
+            "host" => Err(ShellError::InvalidValue {
+                valid: "a non-empty string".into(),
+                actual: format!("'{s}'"),
                 span: value_span,
             }),
-            "scheme" => Err(ShellError::UnsupportedConfigValue {
-                expected: "non-empty string".into(),
-                value: "empty string".into(),
+            "scheme" => Err(ShellError::InvalidValue {
+                valid: "a non-empty string".into(),
+                actual: format!("'{s}'"),
                 span: value_span,
             }),
             _ => Ok(false),