feat(rust/signed-doc): Add 'NotSpecified' variant to Content Encoding validation rule

[saibatizoku]

Description

Closes #528 .

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 428/428}$ | ${\color{red}Fail: 0/428}$ |

[Mr-Leshiy]

LGTM