Add secure pin generation for ui

[somnathb1]

Adds secure random number for PIN
Using the crypto/rand go package for secure random number generation for UI PIN.
Ref: rand package

Also adds a flag --insecure flag to the diagnostics cmd line. When this flag is specified, the insecure random number generation using math/rand package is used. Defaults to false i.e. SRNG.

[AlexeyAkhunov]

Thank you! Could you please modify this to only fallback to the weakrand generation when --insecure flag is provided? If there is an error during the secure generation of the PIN, it should be displayed in a similar way other errors are displayed, otherwise it would be hard to debug.
And another thing - could you please apply gofmt -s -w to the files. I have just realised that this project does not have GitHub actions for code lint and compilation check - I will include this into the list of improvements

[somnathb1]

Removed weakrand on error as a fallback.
Also formatted files.