Refactor value insertions for LogEvent and TraceEvent

[pront]

Action Plan

• Refactor all related calls to use TargetPath.
• Add wrappers that accept Options to avoid code duplication throughout the codebase.
• The VRL string_path feature (currently on by default) should only be used in tests for convenience. See here how to achieve this: https://doc.rust-lang.org/cargo/reference/resolver.html#feature-resolver-version-2.
• Need to update the VRL version used by Vector for the above.