feature(AppApi): Advanced Deploy Options for ExApps

[bigcat88]

This is the AppStore part of this AppAPI feature: nextcloud/app_api#406

This PR adds the ability to specify additional environment parameters, and the AppStore starts emitting information about this via an endpoint.

This is necessary because AppAPI at the stage of pressing the "Deploy and Install" button does not have the application info.xml file, and AppAPI cannot parse the information from there without downloading it - therefore we give it via the standard endpoint.

<environment-variables>
    <variable>
        <name>SOME_VALUE1</name>
        <display-name>Display name 1</display-name>
        <description>Some useful description of it</description>
    </variable>
    <variable>
        <name>SOME_VALUE2</name>
        <display-name>Display name 2</display-name>
        <description>Example of second variable with the 'default' value</description>
        <default>0</default>
    </variable>
</environment-variables>