Design for pre-post backup-restore action types

[Nutrymaco]

Please add a summary of your change

Add design doc after discussion in #6417

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov-commenter]

Codecov Report

Merging #6466 (0392900) into main (e54a8af) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #6466      +/-   ##
==========================================
- Coverage   60.18%   60.18%   -0.01%     
==========================================
  Files         229      229              
  Lines       24215    24219       +4     
==========================================
+ Hits        14575    14577       +2     
- Misses       8632     8634       +2     
  Partials     1008     1008              

see 4 files with indirect coverage changes

[sseago]

Note that we already have an approved design for this (submitted via #4083 and currently located at https://github.com/vmware-tanzu/velero/blob/main/design/new-prepost-backuprestore-plugin-hooks.md)

I haven't reviewed this PR in detail yet, but was it created with reference to that one? If the prior design was insufficient, maybe it would be clearer to submit a PR that modifies that design rather than creates a complete new one.

[Nutrymaco]

@sseago

Yes, that makes sense, I've updated the current design. Сhanged the part about where to call the plugins considering async operations introduced in #5849 and #5993

[anshulahuja98]

I went through the design. I get the overall objective.

Can you also highlight some concrete use cases where users need this. Secondly also look into adding rough Action / Action status structs as part of the design. So we get a better idea of the scope of actions we intend to support.

@Nutrymaco

[Nutrymaco]

1. Use cases:
   • stopping and resuming workload before and after backup
   • starting job after backup for data transfer to DR site
   • notifications about starting and finishing backup/restore
2. Action status struct described here

@anshulahuja98