feat: email i18n

[lfleischmann]

Description

Adds internationalization of emails.

Implementation

• The API now accepts a custom X-Language header on all Flow API endpoints.
• Hanko Elements now transmits the value of the lang attribute of the components as the value for the X-Language header.
• Changing a component's lang attribute updates the language of underlying Hanko client used through a new setter function (only the underlying Flow client is updated since other clients are slated for removal in the near future)
• For relying parties/clients that do not use Hanko Elements: the language can also be provided as an argument to the central Hanko client of the frontend SDK.
• Providing a hyphenated version of language code (+ country code) is now required in order for emails to be correctly translated too - this currently only concerns Brazilian Portuguese. The hyphen is removed when passing the language to the frontend's translation context, so that frontend translations can still be resolved (the keys do not allow hyphens). "Old"-style arguments (i.e. "ptBR") still work but emails will not be translated - up for discussion whether this constitutes a breaking change.
• The webhook token payload for the email.send event now also contains the transmitted X-Language header value in the language claim (the accept_language claim/field of the underlying backend struct has been marked as deprecated in order to be more consistent with the new headers' name and to prevent possible confusion regarding the syntax/semantics of the claim - its value does behave like a HTTP Accept-Language header, it is only one value instead of a list with quality values/weights).

How to test

Startup backend + Mailslurper (or equivalent, configure as required). Build Hanko Elements/Hanko Frontend SDK and use the example "application" in /frontend/elements/src/example.html to toggle languages and request emails. Alternatively, start up one of the examples in /frontend/examples and supply different values for the components' lang attribute.