Change LogEvent::insert function to return a Result<Option<Value>>

[pront]

This is an idea for improving the insertion API. I noticed that insert silently fails when inserting inserting special paths like a-b. See the following test for example:

#[test]
fn special_path_insert() {
    let mut log = LogEvent::default();
    log.insert("a-b", 0); // The path segment is invalid, it needs to be quoted.
    log.insert("\"a-c\"", 1); // Valid path segment.

    assert_eq!(log.get("a-b"), None); } // This returns nothing.
    assert_eq!(log.get("\"a-c\""), Some(&1.into())); // OK

Changing the return type to vector_common::Result<Option<Value>> would require the user to handle a failed insertion. This can prevent subtle bugs. However, this would require a lot of refactoring.

Edit: This is actually only relevant when the string_path VRL crate feature is enabled. This feature is off by default.