feat: added support for custom headers and insecure tls

[mridang]

This pull request adds support for TLS certificate verification control and custom transport headers to the Zitadel Terraform provider.

This changeset adds two new optional provider configuration parameters: insecure_skip_verify_tls allows connecting to ZITADEL instances with self-signed or untrusted TLS certificates for development and testing environments, while transport_headers enables custom headers for both HTTP (authentication) and gRPC (API) requests to support proxy authentication scenarios like GCP IAP.

Example:

provider "zitadel" {
  domain                   = "my-instance.zitadel.cloud"
  insecure_skip_verify_tls = true  # Development environments only
  
  transport_headers = {
    "Proxy-Authorization" = "Bearer ${var.proxy_token}"
  }
  
  jwt_profile_json = file("service-account.json")
}

Closes #228
Closes #259

Definition of Ready

• Short description of the feature/issue is added in the pr description
• PR is linked to the corresponding user story
• Acceptance criteria are met
• All open todos and follow ups are defined in a new ticket and justified
• Deviations from the acceptance criteria and design are agreed with the PO and documented.
• No debug or dead code
• My code has no repetitions
• All non-functional requirements are met
• The generic lifecycle acceptance test passes for affected resources.
• Examples are up-to-date and meaningful. The provider version is incremented.
• Docs are generated.
• Code is generated where possible.

[codecov]

Codecov Report

❌ Patch coverage is 65.85366% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.51%. Comparing base (8fb6b35) to head (5560ad9).
⚠️ Report is 56 commits behind head on main.

Files with missing lines	Patch %	Lines
zitadel/helper/client.go	0.00%	7 Missing ⚠️
zitadel/provider.go	79.41%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #333       +/-   ##
===========================================
- Coverage   57.68%   38.51%   -19.18%     
===========================================
  Files         221      284       +63     
  Lines       25868    30625     +4757     
===========================================
- Hits        14923    11795     -3128     
- Misses       7427    17125     +9698     
+ Partials     3518     1705     -1813     

Flag	Coverage Δ
acceptance-tests	38.51% <65.85%> (-19.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.