Fix saving tags performance

[crazytonyli]

Description

To reproduce the issue:

1. Create a self hosted sites, and add 4000 tags.
2. Use Xcode to launch the Jetpack app, "Enter you existing site address".
3. Go to a post -> Post Settings -> Tags.

From Xcode, you can see the app's CPU usage jumps to 200% and can last for 10 seconds or more.

Testing instructions

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	28173
	Version	PR #24667
	Bundle ID	org.wordpress.alpha
	Commit	579216c
	Installation URL	0vuso9nuidlg0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	28173
	Version	PR #24667
	Bundle ID	com.jetpack.alpha
	Commit	579216c
	Installation URL	6eg70mbuf4re0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[kean]

Nice! So, instead of per-tag queries, it now performs as single fetch request.

Nit: It would've been nice to implement this in Swift and cover with a unit test.

[crazytonyli]

I'll keep the PR as it is for now, since the PostTagService will probably be used very rarely once the "Post Settings -> Add tags" stops using it (see #24670).