Support PulumiPluginJSON.Server in pulumi-java-gen

[t0yv0]

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Issue details

With #709 the responsibility of generating plugin.json with metadata about the package is moving from pulumi-java-gen invocation itself to the generated build system, currently supporting Gradle. One feature lost in translation is populating the server field from PluginDowlnoadURL in the schema.

That is on the old path we have:

		pulumiPlugin := &plugin.PulumiPluginJSON{
			Resource: true,
			Name:     pkg.Name,
			Version:  cfg.Version.String(),
			Server:   pkg.PluginDownloadURL,
		}

We should have the same functionality on the new path. pulumi-java-gen should emit build code that populates PluginDownloadURL from the schema to the generated Java resource plugin.json so the generated SDK code can read it back and act accordingly.

Affected area/feature

[muhlba91]

related to #1109