Upgradeability: use specific storage slot for owner functionality

[K1-R1]

Type of change

• Improvement (refactoring, restructuring repository, cleaning tech debt, ...)

Changes

The following changes have been made:

• The upgradeability lib has been updated to use a specific storage slot for the proxy owner
• The upgradeability example has been updated to use a specific storage slot for the proxy owner
• The upgradeability test contract has been updated to use a specific storage slot for the proxy owner

Notes

The _proxy_owner(), only_proxy_owner() and _set_proxy_owner() functions no longer take storage.proxy_owner as a parameter. Instead they directly read and write to the storage slot 0xbb79927b15d9259ea316f2ecb2297d6cc8851888a98278c0a2e03e1a091ea754 which is sha256("storage_SRC14_1").

Related Issues

Closes #287

Checklist

• [x] I have linked to any relevant issues.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • [x] If my change requires substantial documentation changes, I have requested support from the DevRel team
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.
• [x] I have updated the changelog to reflect the changes on this PR.

[K1-R1]

To be merged after #289