[HttpClient] Add a note about vendor dependency to use URI Templates

[clement-michelet]

URI templates were added with 6.3. However, to use that feature, it requires to install a vendor or add implement your own handler.
Cf. https://github.com/symfony/symfony/blob/6.3/src/Symfony/Component/HttpClient/UriTemplateHttpClient.php#L82

The current documentation is misleading IMHO. I thought it was working out-of-the-box and replaced my simple sprintf with template vars. By knowing there was a dependency requirement, I would not have switch to template vars because it was not worthy.
I discovered it on runtime which is too late and really annoying.

Add a simple note about vendor requirement to use the feature, maybe with a link to packagist search by tag if it's possible.