Refactor/leverage getdecorator api

[jean-michelet]

Implement #99

This change uses the new getDecorator API to reduce module augmentation and improve both type safety and runtime safety.
Reference: https://fastify.dev/docs/latest/Reference/Decorators/#getdecoratort-api

There are other ways to ensure runtime safety with decorators, but using TypeScript, this approach it easier as perform static and runtime checks in one place.

I used Symbol to name decorators, don't know what you think about that.

[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

[jean-michelet]

Started from an existing branch before pulling, which caused this commit history mess.
Will squash commits properly before merging.

[jean-michelet]

I need several feedback on the style, this is why I ask for 3 reviews.

[jean-michelet]

Suggested change

	function createChecker () {
	function createManager () {

[jean-michelet]

Suggested change

	// `request.auth` remains the single source of truth.
	// `request[kAuth]` remains the single source of truth.

[jean-michelet]

@Eomm @mcollina @gurgunday
Not interested in this feature?

I understand that the official demo needs to represent the current state of Fastify's practices.
But it can also be a way for people to discover new features and ways to enjoy Fastify.

Maybe I can add some comments to explain the alternatives to getDecorator api? Like module augmentation?