Buildozer add "rename_if_exist"

Would be great if there was a rename which doesn't error out when attr doesn't exist.

Current workaround:
```
set_if_absent old_attr INVALID
rename old_attr new_attr # NOTE errors out if old_attr is not set
remove_if_equal new_attr INVALID
```

Start by locating the existing rename command and its tests in the Buildozer codebase, then read how it handles a missing attribute. Define the new rename_if_exist behavior from the request and its workaround, and add coverage showing that an absent old_attr does not error while an existing attribute is renamed.

Written by the indexing model from the issue text.