[terraform] Add Custom Download Server Support for Terraform Dev Container Feature

[iTiPo]

This pull request introduces the ability to specify a custom download server for the Terraform Dev Container feature. This enhancement is particularly useful for organizations that maintain internal mirrors or have proxies for HashiCorp downloads, allowing them to control the source of Terraform and Sentinel packages.

Key Changes:

• Added a new option customDownloadServer in the devcontainer-feature.json to allow users to specify an alternative server URL.
• Updated the install.sh script to utilize the custom server URL when provided, ensuring that the download process respects the user's configuration.
• Enhanced documentation in NOTES.md to include guidelines on using the custom download server, along with security considerations to ensure users are aware of potential risks.

Motivation:

• Flexibility: Organizations can now use their own mirrors or proxies, which is essential for environments with restricted internet access or specific compliance requirements.
• Security: By allowing users to specify a trusted server, the feature helps mitigate risks associated with downloading software from untrusted sources.
• Compliance: This change supports organizations that need to adhere to internal policies regarding software distribution and updates.

Testing:

• Updated test scenarios in scenarios.json to include cases for custom download server usage, ensuring that the feature works as expected with different configurations.

[iTiPo]

@microsoft-github-policy-service agree

[Kaniska244]

Changes are fine & test scenarios validated. To be further reviewed by maintainers.