feat: native sbom generation for hcp

[anurag5sh]

Description

This PR adds the native SBOM generation capability within Packer in the existing hcp-sbom provisioner.
Packer will automatically download syft binary for generating the sbom. This also supports using a custom binary of your choice to generate the sbom.

Relavant config options has been added to support this feature.

[galapas1]

Thanks for the native SBOM generation feature. Summary of requested changes:

• Docs: Clarify that source and auto_generate are mutually exclusive.
• Validation: Consider allowing elevated_user/elevated_password when using source so users can toggle only auto_generate without clearing those fields.
• Structure: Extract the retry callback into a named function for readability and testability.
• Syft version: Consider pinning to a known version with Dependabot, or document the trade-off of using "latest".
• Windows cleanup: Quote paths in del for paths with spaces.
• Packer core: Consider delegating FlatConfig() to the inner provisioner for consistency.

[anurag5sh]

Thanks for the native SBOM generation feature. Summary of requested changes:

• Docs: Clarify that source and auto_generate are mutually exclusive.
• Validation: Consider allowing elevated_user/elevated_password when using source so users can toggle only auto_generate without clearing those fields.
• Structure: Extract the retry callback into a named function for readability and testability.
• Syft version: Consider pinning to a known version with Dependabot, or document the trade-off of using "latest".
• Windows cleanup: Quote paths in del for paths with spaces.
• Packer core: Consider delegating FlatConfig() to the inner provisioner for consistency.

I have made all the changes as requested, please take another look at it whenever possible. Thanks!
@galapas1 @tanmay-hc

[anurag5sh]

Oh and here's the docs for this, since we moved out to unified repo now - hashicorp/web-unified-docs#1999

[galapas1]

Approved with requested edits shared via slack.