API evolution tutorial

Signed-off-by: arhimede <julian@dotkernel.com>

Author: marioradu05

docs/book/v5/tutorials/api-evolution.md

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+# API Evolution pattern
+
+API evolution: Updating an API while keeping it compatible for existing consumers by adding new features, fixing bugs, planning and removing outdated features.
+
+## How it works
+
+In DotKernel API we can mark an entire endpoint or a single method as deprecated using attributes on handlers.
+We use response headers to inform the consumers about the future changes by using 2 new headers: **link** and **sunset**.
+
+1) `Link` - it's a link to the official documentation pointing out the changes that will take place.
+2) `Sunset` - this header is a date, indicating when the change will roll out.
+
+**Both headers are independent, you can use them separately.**
+
+> Make sure you have the `DeprecationMiddleware:class` piped in your `pipeline` list. In our case it's `config/pipeline.php`.
+
+### Marking an entire endpoint as deprecated
+
+When you want to mark an entire resource as deprecated you have to use the ``ResourceDeprecation`` attribute.
+
+```php
+...
+#[ResourceDeprecation(
+    sunset: '2038-01-01',
+    link: 'https://docs.dotkernel.org/api-documentation/v5/core-features/versioning',
+    deprecationReason: 'Resource deprecation example.',
+    rel: 'sunset',
+    type: 'text/html'
+)]
+class HomeHandler implements RequestHandlerInterface
+{
+...
+```
+
+In the example above, the ``ResourceDeprecation`` attribute is attached to the class, marking the entire `/` (home) endpoint as deprecated starting from `2038-01-01`.
+
+Running the following curl will print out the response headers where we can see the **Sunset** and **Link** headers.
+
+```shell
+curl --head -X GET http://0.0.0.0:8080 -H "Content-Type: application/json"
+```
+
+```shell
+HTTP/1.1 200 OK
+Host: 0.0.0.0:8080
+Date: Mon, 24 Jun 2024 10:23:11 GMT
+Connection: close
+X-Powered-By: PHP/8.2.20
+Content-Type: application/json
+Permissions-Policy: interest-cohort=()
+Sunset: 2038-01-01
+Link: https://docs.dotkernel.org/api-documentation/v5/core-features/versioning;rel="sunset";type="text/html"
+Vary: Origin
+```
+
+### Marking a method as deprecated
+
+Most of the time you want to deprecate only an endpoint, so you will need to use the `MethodDeprecation` attribute which has the same parameters, but it attaches to a handler method.
+
+```php
+...
+class HomeHandler implements RequestHandlerInterface
+{
+    ...
+
+    #[MethodDeprecation(
+        sunset: '2038-01-01',
+        link: 'https://docs.dotkernel.org/api-documentation/v5/core-features/versioning',
+        deprecationReason: 'Method deprecation example.',
+        rel: 'sunset',
+        type: 'text/html'
+    )]
+    public function get(): ResponseInterface
+    {
+        ...
+    }
+}
+```
+
+Attaching the `MethodDeprecation` can only be done to HTTP verb methods (`GET`, `POST`, `PUT`, `PATCH` and `DELETE`).
+
+If you followed along you can run the below curl: 
+
+```shell
+curl --head -X GET http://0.0.0.0:8080 -H "Content-Type: application/json"
+```
+
+```shell
+HTTP/1.1 200 OK
+Host: 0.0.0.0:8080
+Date: Mon, 24 Jun 2024 10:54:57 GMT
+Connection: close
+X-Powered-By: PHP/8.2.20
+Content-Type: application/json
+Permissions-Policy: interest-cohort=()
+Sunset: 2038-01-01
+Link: https://docs.dotkernel.org/api-documentation/v5/core-features/versioning;rel="sunset";type="text/html"
+Vary: Origin
+```
+
+### NOTES
+
+> If `Link` or `Sunset` do not have a value they will not appear in the response headers.
+
+> `Sunset` has to be a **valid** date, otherwise it will throw an error.
+
+> You **cannot** use both `ResourceDeprecation` and `MethodDeprecation` in the same handler.
+
+> Deprecations can only be attached to handler classes that implement `RequestHandlerInterface`.
+
+> The `rel` and `type` arguments are optional, they default to `sunset` and `text/html` if no value was provided and are `Link` related parts.