Improve API for setting metadata version of delegated roles

[joshuagl]

The repo API includes functions for setting the version () of top-level metadata through:

func (r *Repo) SetTimestampVersion(v int64) error {}
func (r *Repo) SetRootVersion(v int64) error {}
func (r *Repo) SetTargetsVersion(v int64) error {}
func (r *Repo) SetSnapshotVersion(v int64) error {}
func (r *Repo) SetTimestampVersion(v int64) error {}

However, we do not yet provide any API for modifying the version of delegated targets roles. This absence of API results in go-tuf users having to implement functions which modify the version number in a file IFF that file isn't a top-level role, for example: sigstore/root-signing#417

We should provide a function to set the version number in a delegated targets role metadata, something like:

func (r *Repo) SetDelegatedTargetsVersion(metaName string, v int64) error {}

[znewman01]

Related: #330

[asraa]

How related do you think this is to a sign command that updates the expiration and therefore the version?

That's the main use-case for this command: a metadata refresh near expiration.

[znewman01]

I'm mostly referring to:

I think we should have separate UpdateDelegatedRole... functions for modification.

Originally posted by @ethan-lowman-dd in #330 (comment)

I think it's worth considering any proposed changes to the interface for interacting with delegations together.

[asraa]

I think we should have separate UpdateDelegatedRole... functions for modification.

I think versions & expiration are separate from properties of delegated roles: I think I wouldn't use UpdateDelegatedRole to a new version, but I might change the role prpoerties itself. Versions and expirations are properties of refreshing or updating all types of metadata, which is why I see them as different

:/ Ahhhhh how I wish there were repository API models!

[rdimitrov]

Closing since the code base changed and this is no longer valid.

Thanks for raising this 👍