Fix OmegaConfigLoader __repr__ and missing keys method

[noklam]

Description

Fix #3618, since we are trying to improve the interactive experience. While it's still a bit unclear as AbstractConfigLoader inherit from UserDict, OmegaConfigLoader only presents traits for dictionary access pattern, i.e. dict["key"], but not the other API such as iterating the keys.

Development notes

The PR is not a perfect fix, but nonetheless address two issues:

1. __repr__ is misleading as it is missing some arguments to start with, to do this I also have to save self._custom_resolver)
2. keys is implemented so that at least it is iterable, we can consider __iter__ and values as well but I want to keep this PR minimal.

Before change

After change

Developer Certificate of Origin

We need all contributions to comply with the Developer Certificate of Origin (DCO). All commits must be signed off by including a Signed-off-by line in the commit message. See our wiki for guidance.

If your PR is blocked due to unsigned commits, then you must follow the instructions under "Rebase the branch" on the GitHub Checks page for your PR. This will retroactively add the sign-off to all unsigned commits and allow the DCO check to pass.

Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[noklam]

I get this error when the dictionary return a list[str], I don't know why this is important as I found it annoying you always have to do list(keys) to see the real content (unlike dict.keys()`, but this seems to be what Python suggest to do so I am following the convention here. https://docs.python.org/3/library/stdtypes.html#dictionary-view-objects

mypy kedro --strict --allow-any-generics --no-warn-unused-ignores
kedro/config/omegaconf_config.py:267: error: Return type "list[str] | None" of "keys" incompatible with return type "KeysView[Any]" in supertype "Mapping" [override]

As a result it shows:

In [25]: KeysView(c.config_patterns)
Out[25]: KeysView({'catalog': ['catalog*', 'catalog*/**', '**/catalog*'], 'parameters': ['parameters*', 'parameters*/**', '**/parameters*'], 'credentials': ['credentials*', 'credentials*/**', '**/credentials*'], 'globals': ['globals.yml']})

In [26]: p = c.config_patterns

In [27]: list(KeysView(p))
Out[27]: ['catalog', 'parameters', 'credentials', 'globals']

I don't find this particular valuable, but this seems to be the only way to please mypy,

[ElenaKhaustova]

Approving with a question, the rest LGTM!