Add functions: add/update/delete Targets in TrustedRoot

[fghanmi]

Summary

The purpose of this PR is to add the required changes to add and update targets in the TrustedRoot. This will involve implementing two new functions:
add_target(&mut self, new_target: TargetType, target_name: Target) -> Result<()>
update_target(&mut self, target: TargetType, target_name: Target) -> Result<()>

These new functions improve the flexibility and usability of the TrustedRoot, making it easier to manage targets effectively.

Related issue: #384

Release Note

Feature: Add and Update Targets in TrustedRoot

Documentation

[fghanmi]

Hello @flavio, @danbev,
Could you please have a look ?
Thank you in advance!

[flavio]

Hi @fghanmi,
sorry for the delay, I was on vacation.

Can you provide some examples about why we would need to offer the ability to alter the contents of the TUF repository?

Currently the code cares only about downloading the official TUF repository or consuming a local checkout, then rely on that to perform different checks.

[fghanmi]

Hi @fghanmi, sorry for the delay, I was on vacation.

Can you provide some examples about why we would need to offer the ability to alter the contents of the TUF repository?

Currently the code cares only about downloading the official TUF repository or consuming a local checkout, then rely on that to perform different checks.

Hello @flavio
actually, on our side we need to be able to manipulate trust_root.json in another Rust tool: tuftool (https://github.com/awslabs/tough/tree/develop), since as of now, only sigstore-rs manages the new-style targets trust_root.json. So, it's the library where this code would fit and would be reusable by other people who have a similar usecase.
In this case, we would be able to use this crate in tough/tuftool to manipulate trust_root.json.
(a similar change was done on sigstore-go sigstore/sigstore-go#247)

[flavio]

I think changing the contents of the TUF repository is out of the scope of this library. Hence closing.

I can help with getting these changes into the tough crate upstream if you want