Add .gitattributes file to generated SDKs (#1949)

The java version of pulumi/pulumi#21034

Generate a .gitattributes file for SDKs that marks the SDK code as
“generated”, and lets for example GitHub collapse changes to these files
by default in the PR view.

Author: julienp

.changes/unreleased/Improvements-1949.yaml

@@ -0,0 +1,6 @@
+component: codegen
+kind: Improvements
+body: Add .gitattributes file to generated SDKs
+time: 2025-12-09T12:44:31.554497+01:00
+custom:
+    PR: "1949"

pkg/cmd/pulumi-language-java/testdata/sdks/any-type-function-15.0.0/.gitattributes

@@ -2288,6 +2288,9 @@ func GeneratePackage(
 
 	// Generate each module.
 	files := fs{}
+
+	files.add(".gitattributes", codegen.GenGitAttributesFile())
+
 	for p, f := range extraFiles {
 		files.add(p, f)
 	}