[Suit] Propose to add suit-uninstall in Multiple Trust Domains draft

[Ken Takayama <[e-mail]>]

Dear SUIT WG,

I'm Ken Takayama an implementer of SUIT and TEEP parser and encoder.
Before WG Last Call, I want to make some comments to
draft-ietf-suit-trust-domains.

- I could encode depending manifest using dependency
  + which is now explained in [teep-protocol v09](
https://datatracker.ietf.org/doc/html/draft-ietf-teep-protocol-09#section-4.4.3)
and
  + the binary is in the Appendix E example 3, thank you Brendan for your
comments in the past IETF meeting
- I could write its parser resolving dependency
  + the functions are enough when installing depending manifests
- The suit-dependency-resolution value needs to be updated
  + see [Issue #70](https://github.com/suit-wg/manifest-spec/issues/70) in
detail
- I propose adding suit-uninstall to the draft (see below)


# Proposal
Add suit-uninstall to the Multiple Trust Domains document
because it is useful functionality when updating depending components in
some situations.
When updating a dependent component the SUIT Manifest parser decrements
the reference counter of dependee's manifest (such as library) in the old
versoin.
If a component is not required anymore, the SUIT Manifest parser MAY
uninstall it
by executing commands listed in suit-uninstall.


## Example situation: Updating an application
Currently Installed
  + component ["application"] version 0
    * depends component ["library-a"] version 0
  + component ["library-a"] version 0

  |
  v

To be Updated
  + component ["application"] version 1
    * depends component ["library-b"] version 0
  + component ["library-b"] version 0


## Issue
The library-a is no more required and should be uninstalled,
but how to uninstall it when updating the application?

This problem is complicated at some points.
- There might be no uninstall (or unlink) manifest of library-a
  + because the library-a developer does not want to uninstall it,
    just the application does not require it anymore
- The library-b manifest may not know how to uninstall library-a
  + when library-a and library-b are created by other parties
- The application manifest version 1 may unlink the library-a
  + but the library-a manifest would remain in the device
  + the library-a would be doubly unlinked when the library-a developer
creates uninstall manifest


## Detailed proposal
The library-a manifest stores how to uninstall the component itself.
When updating the application, the SUIT Manifest parser decrements
reference counters of ["application"] version 0 and ["library-a"] version 0
respectively.
The counters becomes 0 and the SUIT Manifest parser MAY uninstall
the library-a by executing commands listed in suit-uninstall and
delete its manifest finally.


Best,
Ken