Support confidential EVM proxy deployments

[aefhm]

Is your feature request related to a problem? Please describe.
Current default proxy relies on a getStorageAt which does not work with confidential EVMs such as Sapphire.

Describe the solution you'd like
An integrated way to allow upgrades on confidential EVMs. #525 is one suggestion by using an exposed owner function.

Describe alternatives you've considered
An alternative would be to rewrite the proxy contract to conditionally check msg.data and compare against a list of values such as 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103 and return the owner as requested.

Additional context
N/A. See proposed solution #525.

Happy to contain discussion on this issue or the suggested PR. Thanks for looking!