backend/http: add ca_file support for custom CA trust

[vincent-turato]

Summary

Add support for configuring a custom CA certificate bundle for the HTTP backend using a new ca_file option (and TF_HTTP_CA_FILE).

This allows TLS certificate verification with private or internal CAs without disabling verification.

Fixes #36937

Changes

• Add ca_file / TF_HTTP_CA_FILE backend option
• Load CA certificates from file and use them for TLS verification
• Merge file-based CAs with existing inline CA configuration
• Add unit tests for CA loading and error handling

Target Release

1.15.x

Changes to Security Controls

Yes. This change improves TLS security by allowing users to configure a trusted CA bundle for the HTTP backend. This enables proper server certificate verification in environments that use private or internal certificate authorities, reducing the need to disable TLS verification via skip_cert_verification.

No existing security controls are removed or weakened.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.

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[hashicorp-cla-app]

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