Add UsedImplicitly to IDalamudPlugin

[ionite34]

This adds the UsedImplictly Annotation to IDalamudPlugin to clear up warnings regarding plugin classes not being instantiated.

[KazWolfe]

Does this actually work if your plugin doesn't have JetBrains.Annotations as a dependency? I thought all of these annotations were stripped by the compiler.

[ionite34]

Does this actually work if your plugin doesn't have JetBrains.Annotations as a dependency? I thought all of these annotations were stripped by the compiler.

Actually yeah since the Release builds of Dalamud are used for development this is going to be stripped. (I noticed the existing MeansImplicitUse attribute on [PluginService] doesn't work either). Whether or not the consumer adds the Jetbrains.Annotations dependency the attributes are already stripped and we can't get them back without rebuilding.

The other options are either

1. Including Jetbrains.Annotations in release builds and compiling with JETBRAINS_ANNOTATIONS condition
2. Adding a xml external annotations file https://www.jetbrains.com/help/resharper/Code_Analysis__External_Annotations.html and either add it to the release output or make a separate NuGet Package that is referenced by the Dalamud SDK.

I think the xml external annotations file is probably lightweight enough to include in release output for simplicity, and it seems we only need a couple of annotations for implicit use for IDalamudPlugin and the IoC attributes.

For the IDalamudPlugin it would look like this, with similar structure for the IoC injection attributes:

<?xml version="1.0" encoding="utf-8"?>
<assembly name="Dalamud">
    <!-- Mark IDalamudPlugin as implicitly used -->
    <member name="T:Dalamud.Plugin.IDalamudPlugin">
        <attribute ctor="M:JetBrains.Annotations.UsedImplicitlyAttribute.#ctor(JetBrains.Annotations.ImplicitUseKindFlags,JetBrains.Annotations.ImplicitUseTargetFlags)">
            <argument>8</argument>
            <argument>4</argument>
        </attribute>
    </member>
</assembly>

I can change this PR to add the external annotations xml instead if that is acceptable