Add missing lifetime annotation to `EscapedStr::as_escaped_str` (#3920)

`JsonTokenIterator` parses and returns `Token`s from a provided u8
slice. `EscapedStr` is used as a member field in several token variants,
such as `ObjectKey` and `ValueString`. While `as_escaped_str` returns a
reference to the underlying `&'a str`, the lifetime annotation was
missing from its return type.

This PR adds the missing lifetime annotation.

Co-authored-by: Fahad Zubair <fahadzub@amazon.com>