refactor(rust/signed-doc): Move validate_signatures as a separate validation rule

[apskhem]

Description

• Added a new module signature.rs for SignatureRule.
• Moved validate_signatures as a part of configurable SignatureRule
• Moved validate_signature to signature.rs
• Added trait constraint VerifyingKeyProvider to Rules as required by validate_signature
• Updated unit tests, merged two dummy providers into one

Related Issue(s)

Closes #498

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 | ${\color{lightgreen}Pass: 423/423}$ | ${\color{red}Fail: 0/423}$ |

[Mr-Leshiy]

other than few small comments LGTM

[Mr-Leshiy]

LGTM