refactor: move watch functions to dedicated file

[jooola]

This adds a clear separation between the actions API client and the actions watch functions.

[codecov]

Codecov Report

Attention: Patch coverage is 84.88372% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 80.43%. Comparing base (87149f3) to head (2304143).

Files	Patch %	Lines
hcloud/action_watch.go	84.88%	12 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #423   +/-   ##
=======================================
  Coverage   80.43%   80.43%           
=======================================
  Files          32       33    +1     
  Lines        5756     5756           
=======================================
  Hits         4630     4630           
  Misses        718      718           
  Partials      408      408           

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[github-actions]

The backport to release-1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-1.x release-1.x
# Navigate to the new working tree
cd .worktrees/backport-release-1.x
# Create a new branch
git switch --create backport-423-to-release-1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 575c24c57e5a38c48ca747864753b43cd7a8261b
# Push it to GitHub
git push --set-upstream origin backport-423-to-release-1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-1.x

Then, create a pull request where the base branch is release-1.x and the compare/head branch is backport-423-to-release-1.x.