Don't overwrite files if file exists unless explicitly indicated by user

[iamsamirzon]

This issue is created to replace a finding in #93. Refer example here . #83 (comment)

Desired Resolution

Search through all code in Notation, Notation-go, Notation-go-core and before creating a new file, check for its existence before overwriting it. And unless specifically asked by the user don't overwrite it.

[shizhMSFT]

Went through all existing code. Currently, we are not overwriting any files except updating config files.

The original comment was there for docker plugins. However, those plugins have already been removed.

[shizhMSFT]

Reopen the issue as notation sign has an --output option for local signing and may overwrite file. However, I think we don't need user confirmation as per common Unix / Linux experience.