Allow to set form-action for CSP header

[dwendland]

Proposed changes

Modern browsers like Chrome follow the Content-Security-Policy directives. With the current version of helmet this is set to 'self' only: https://github.com/helmetjs/helmet/blob/main/CHANGELOG.md#changed-3 Due to this, the forwarding to callback URLs during authorization is blocked by the browsers, if the host of the callback URL is not specified as form-action in the CSP header.

This change allows to configure the form-action parameter. If nothing is configured, it will still use the default of helmet.

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[dwendland]

Also added option to overwrite CSP script-src directive

[aalonsog]

Thanks and apologies for the delay. We are working on this and will merge during next week