feat(server): add screenshot command

[jooola]

Adds a new hcloud server screenshot -o screenshot.png <server>, that take a screenshot of the server console using a VNC client.

Fixes #1420

[jooola]

Still missing from this PR:

• Finalize the e2e test.
• Replace slog logger with what the CLI is using.
• Hide log messages from the go-vnc library.
• Check if another fork og go-vnc is better that the one from alexnet, I saw other forks being a bit more active.
• Maybe golang.org/x/net/websocket is not the best library, but it does not add a new dependency. Check if this is important.

[codecov]

Codecov Report

❌ Patch coverage is 10.47619% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.21%. Comparing base (f37b5fa) to head (885dcb8).

Files with missing lines	Patch %	Lines
internal/cmd/server/screenshot/screenshot.go	0.00%	78 Missing ⚠️
internal/cmd/server/screenshot.go	38.46%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1435      +/-   ##
==========================================
- Coverage   72.80%   72.21%   -0.59%     
==========================================
  Files         300      302       +2     
  Lines       11104    11209     +105     
==========================================
+ Hits         8084     8095      +11     
- Misses       2120     2214      +94     
  Partials      900      900              

Flag	Coverage Δ
e2e	50.86% <10.47%> (-0.40%)	⬇️
unit	67.20% <10.47%> (-0.54%)	⬇️

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[apricote]

I would prefer to mark this new command as experimental initially.

[apricote]

Would be awesome if users could directly show it in their terminal.

For example with the Kitty graphics protocol I can run this right now to render the image in the terminal (Ghostty):

hcloud server screenshot <name>
viu screenshot.png

If -o - would mean stdout:

hcloud server screenshot <name> -o - | viu -

[phm07]

The server message channel should be closed or we might end up waiting for it endlessly.

Suggested change

	go func() {
	go func() {
	defer close(vncCfg.ServerMessageCh)

[phm07]

This overwrites screenshot.png in the current dir without asking. Should we fail if the file already exists and add an explicit --overwrite flag?