feat: Sign the full SAML response and attach the Signature

[ttradesman]

We want to integrate the AWS Client VPN with Zitadel via SAML2 as the SSO solution to login to the VPN. However, as described in the AWS docs: "The SAML assertion and SAML documents must be signed." – and Zitadel only delivers a signed assertion in its response.

These code changes fixes this issues, and successfully signs the full response.

Reference: https://discord.com/channels/927474939156643850/1389861642854072343/1389861642854072343

See issue #103.

Definition of Ready

• I am happy with the code
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• PR is linked to the corresponding user story
• Acceptance criteria are met
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• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
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• Where possible E2E tests are implemented
• Documentation/examples are up-to-date
• All non-functional requirements are met
• Functionality of the acceptance criteria is checked manually on the dev system.

[livio-a]

Sorry for the late reply. I'll look into this by the end of this week.

[elinashoko]

hey @ttradesman firstly, thank you for your contribution and also, very sorry for the delay. Livio is on holiday at the moment, so we'll assign the review to another engineer. Thank you for the patience!

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.47%. Comparing base (524a649) to head (9b82f27).
⚠️ Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/provider/post.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
+ Coverage   47.36%   50.47%   +3.10%     
==========================================
  Files          21       26       +5     
  Lines        1974     2001      +27     
==========================================
+ Hits          935     1010      +75     
+ Misses        981      920      -61     
- Partials       58       71      +13     

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[ghost]

🎉 This PR is included in version 0.4.0 🎉

The release is available on GitHub release

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