[PYG-394] ☺ Fix avoid appending to existing (#489)

# Description

Currently when pygen appends to the `ClientName` it can lead to a large
header if the client is initialized multiple times.

## Bump

- [x] Patch
- [ ] Minor
- [ ] Skip

## Changelog
### Fixed

- [alpha] The `QueryExecutor` no longer prefix the `client_name` in the
`CogniteClient` if it is already set.
