Refactor vertices conversion to reduce clones and clean up test code. Remove RootLabel name usages

[farost]

Usage and product changes

We reduce the amount of methods that consume self and force us to use clone for their usages:

• Move fn vertex from TypeAPI to TypeVertexEncoding
• Make some non-into_``TypeEdgeEncoding methods accept &self instead of self.

Moreover, we clean the test code that used RootLabel, switching to Kind instead.

[typedb-bot]

PR Review Checklist

Do not edit the content of this comment. The PR reviewer should simply update this comment by ticking each review item below, as they get completed.

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Trivial Change

• [ ] This change is trivial and does not require a code or architecture review.

Code

• [ ] Packages, classes, and methods have a single domain of responsibility.
• [ ] Packages, classes, and methods are grouped into cohesive and consistent domain model.
• [ ] The code is canonical and the minimum required to achieve the goal.
• [ ] Modules, libraries, and APIs are easy to use, robust (foolproof and not errorprone), and tested.
• [ ] Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
• [ ] The code is algorithmically efficient and scalable for the whole application.

Architecture

• [ ] Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
• [ ] Any required build and release automations are updated and/or implemented.
• [ ] Any new components follows a consistent style with respect to the pre-existing codebase.
• [ ] The architecture intuitively reflects the application domain, and is easy to understand.
• [ ] The architecture has a well-defined hierarchy of encapsulated components.
• [ ] The architecture is extensible and scalable.

[farost]

Will revive it after one more pr next week.

[farost]

Consumed by https://github.com/typedb/typedb/pull/7157