feat(rust/signed-doc): Add chain field metadata validation rule

[apskhem]

Description

Thanks for contributing to the project!
Please fill out this template to help us review your changes.

Related Issue(s)

Closes #539

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 | ${\color{lightgreen}Pass: 527/527}$ | ${\color{red}Fail: 0/527}$ |

[Mr-Leshiy]

Dont forget to also implement ChainRule::new method for initialisation of the ChainRule from the DocSpecs

[Mr-Leshiy]

LGTM