Add attributes to metadata XSD file

[scollado]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	N/A
License	MIT
Doc PR	TBD

Not a critical thing, but as I tend to prefer XML for configuration files I'm sure other will enjoy the change :
Added attributes allow to validate resources.xml files as developers can do with other Symfony config files (ex: routing, services, ...)
This would ease auto-completion by editors supporting XML structure hints from the XSD (like PHPStorm) by defining the namespace in the XML header, as other XML config files do in Symfony :

<!-- src/AppBundle/Resources/config/api_resources/resources.xml -->

<?xml version="1.0" encoding="UTF-8" ?>
<resources xmlns="https://api-platform.com/schema/metadata"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="https://api-platform.com/schema/metadata
           https://api-platform.com/schema/metadata/metadata-2.0.xsd">
   [...]
</resources>

The proposed URI for the schema (https://api-platform.com/schema/metadata/metadata-2.0.xsd) is subject of discussion as it implies that the last stable version of the XSD file for the choosen branch is publicly available.

I'll make the PR to the Getting-Started page of the documentation once those details will be sorted out.
WHat do you think about this ?

[scollado]

I did not commit updated fixtures as the XSD is not yet available from a public URL, I'll update the PR when the final location will be chosen (or is this not required ? I tried locally by defining a valid internal URL for the xsd file).

[dunglas]

Great. But as with Symfony, it should not rely on nor request an external server in production. So you can add fixtures without the file being publicly available, it will be a good test that it works.

[scollado]

@dunglas Updated fixtures & exception message regexp in tests to include proposed namespace (after messing up with my git remotes while updating 😕 ).
Forking the documentation right-now to prepare a PR including the changes.

[dunglas]

Thank you @scollado

[dunglas]

This PR breaks tests: https://travis-ci.org/api-platform/core/jobs/226752482

Do you have an idea of the problem @scollado?

[soyuka]

ping @scollado would you be able to patch the documentation about XML declarations there?

[soyuka]

@dunglas fixed, it was only an issue on master where we had more tests on XML than on 2.0.

[jakesylvestre]

As per #1229, does this need to be done to the filter.xml file as well?

[jakesylvestre]

Nevermind. #1229 is fixed.