[Form] Automatically add step attribute to HTML5 time widgets to display seconds if needed
bug #10777 [Form] Automatically add step attribute to HTML5 time widgets to display seconds if needed (tucksaun)

This PR was squashed before being merged into the 2.3 branch (closes #10777).

Discussion
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[Form] Automatically add step attribute to HTML5 time widgets to display seconds if needed

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9976, #10203
| License       | MIT
| Doc PR        | none

Same issue as #9976 and #10203: when you add a `time` field to a form with options `single_text` (so HTML5) and `with_seconds`, the generated input does not contain the `step` attribute, therefore the browser does not show them, leading to an error at the submit because of an invalid format.

Compared to #9976/#10203:
* Unit testable
* Available directly in the component
* Available in other templating format than twig
* Still able to customise the step attribute by hand

Commits
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a379298 [Form] Automatically add step attribute to HTML5 time widgets to display seconds if needed