[5.x] Ensure default config values are available in form tag

[duncanmcclean]

This pull request fixes an issue introduced by #9685, where the {{ fields }} array in the form tag doesn't include any fields where the values should be falling back to the fieldtype's defaults.

Example

For example, if you have a text field on your form blueprint and have the "input type" set to text, {{ input_type }} wouldn't output anything on the frontend since the default doesn't actually get saved to the field's config array.

However, if you were to change the "input type" for that field to email, the {{ input_type }} would return correctly on the frontend since the value is being explicitly set in the field's config array.

This PR fixes the issue by merging in the fieldtype's default config values with the field's config values.

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Fixes #10077.

[jasonvarga]

Why not just make Field::toArray() include the defaults?

[duncanmcclean]

Why not just make Field::toArray() include the defaults?

I wasn't sure if that'd cause unintended side affects somewhere or not. I've moved it into the Field::toArray() method.