chore(path): make path reading api more rusty (#9103)
### Description
Remove `read_existing_to_string_or` in favor of
`read_existing_to_string` to consolidate on the more Rust-y API for
reading a path to a file.
`read_existing_to_string_or` is an awkward API due:
- It conflates two outcomes: The file not existing and the file existing
with the contents of the default
- The default is provided as `Result<impl Into<String>, io::Error>`.
`impl Into<String>` was chosen so unnecessary work could be avoided.
This is limiting compared to `FnOnce() -> String`.
- It prevents us from using the battle tested `Option` API
The benefits of not conflating the two outcomes is best displayed when
trying to read a config file. Instead of defaulting having a default of
`{}` and then parsing it as JSON we can skip the parse and just use
`ConfigurationOptions::default()`
### Testing Instructions
Existing unit tests + 👀
