feat: automatically generated instanceId

[nunogois]

https://linear.app/unleash/issue/2-2395/make-instance-id-autogenned-go

This makes instanceId an automatically generated property, instead of being set by the user.

Similar to:

• https://github.com/Unleash/unleash-client-ruby/pull/179
• https://github.com/Unleash/unleash-client-node/pull/640
• https://github.com/Unleash/unleash-client-dotnet/pull/226