feat: make CInput events transparent

[bastianjoel]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
This PR makes the events of the native input element accessible via the CInput component.

[woothu]

Hi! Thank you for submitting this interesting idea!

What events besides 'input' and 'change' are useful in your opinion? (Use cases)

[bastianjoel]

@woothu I'm using the blur and keypress.enter event very often to save single-input forms for example.

Bootstrap-Vue uses this technique btw for their inputs too. (https://github.com/bootstrap-vue/bootstrap-vue/blob/bba1dc80bec6922d42e0ec43fa17f1a29013ef0f/src/components/form-input/form-input.js#L159)

[woothu]

It seems like a useful feature. Thank you for contributing!

I have added this feature also in other form components 97085df