refactor: move checksum calculation to utils

[h0lyalg0rithm]

Proposed changes

Move sha256 calculation to utils.go file.
Add tests to validate change

Types of changes

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• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices applies)

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[glimchb]

@h0lyalg0rithm this is great, please fix small gosec issue

  ::medium file=sztp-agent/pkg/secureagent/utils.go,line=72,col=12::G304: Potential file inclusion via variable (gosec)

[glimchb]

@h0lyalg0rithm in a separate PR , not here, wdyt to move downloadAndValidateImage(), copyConfigurationFile(), launchScriptsConfiguration() to a new file... ? it looks not belonging to daemon.go

[h0lyalg0rithm]

@h0lyalg0rithm in a separate PR , not here, wdyt to move downloadAndValidateImage(), copyConfigurationFile(), launchScriptsConfiguration() to a new file... ? it looks not belonging to daemon.go

I think we should update the golinter version as it displays the line with the issue in the github action run.Here is the output
https://github.com/h0lyalg0rithm/sztp/actions/runs/10222129019/job/28286107496

[h0lyalg0rithm]

@h0lyalg0rithm in a separate PR , not here, wdyt to move downloadAndValidateImage(), copyConfigurationFile(), launchScriptsConfiguration() to a new file... ? it looks not belonging to daemon.go

Yes it makes sense to move them

[glimchb]

I think we should update the golinter version

sure, go for it