Add logic to upload the Godot Android library to MavenCentral

Add environment variables to sign the release build for the Godot
Android editor and to publish the library to MavenCentral.
If the environment vars are not defined, we do a simple unsigned
`release_debug` build for the Android editor.

Change `config.sh.in` template to use single quotes by default, to
prevent expanding special characters in environment variables.

To publish to MavenCentral, a new `build-android/upload-mavencentral.sh`
script is added. It needs to run after the build using gradle, but we
still want it to be optional and used only when making an official
release, so we copy the compiled sources in the first step.

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>

.gitignore

@@ -1,5 +1,6 @@
-# User-specific configuration and signing key
+# User-specific configuration and signing keys
 config.sh
+*.jks
 *.pfx
 *.pkcs12