[keycloak-dev] Theme resource provider - ability for custom authenticators to load additional templates

I was experimenting with a custom authenticator and found it annoying that I had to copy templates manually to the themes directory. To resolve this I added a Theme Resource Provider SPI. A theme resource provider can provide additional templates and/or resources alongside the custom authenticator. Themes are always searched first for templates and resources. Template resource providers are only searched if not found in themes. This is important as it allows a theme to override the templates and/or resources provided by a theme resource provider. The code is ready in the PR https://github.com/keycloak/keycloak/pull/4971 For an example on how it's used take a look at https://github.com/stianst/keycloak-experimental/tree/master/u2f.