Add support for plain Symfony forms and validation

[ggioffreda]

By defaulting to the submitted form value this validator can be used with custom Symfony form types.

This is useful for example:

• when it's not possible to post to the hardcoded "cf-turnstile-response" form field
• when using a third party widget (like vue-turnstile), while still performing server side validation with Symfony validators
• when the data is not submitted through a standard web request

[matthieu2607]

Hi @ggioffreda,

Thank you for your contribution.

I will consider your contribution quickly ;)

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[fliespl]

I second this PR - currently it's hardcoded to field name: cf-turnstile-response, so example with "security" field name doesn't work.