feat(rust/signed-doc): signed doc metadata serde refactoring

[no30bit]

Description

Removes InnerMetdata. Maintains serde::Deserialize impl. Extends Metadata api with from_fields constructor.

Related Issue(s)

Closes #366

Description of Changes

• Remove InnerMetadata.
• Implement Serialize for supported_field types.
• Implement Metadata::from_json – replacing previous impl with InnerMetadata.
• Add Metadata::from_fields that does the same as Metadata::from_metadata_fields used to do, but accepts a vector of SupportedField.

Breaking Changes

No.

Related Pull Requests

#338

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 | ${\color{lightgreen}Pass: 351/351}$ | ${\color{red}Fail: 0/351}$ |

[Mr-Leshiy]

LGTM