feat: Add option to inject dynamic template component

[DominicBoettger]

It should be possible to add a page template or to fall back to a default one.

Currently there is only some default set which renders the header informations and a default set of components into the html body.

The template should itself be webcomponent.

The current render function in the site loader:

render() {
    // should stay here to remove old headers
    this.renderHeader();
    this.renderHtmlLangTag();
    if (this.data && this.data.data) {
      const output = [];
      if (this.data.menu && this.data.menu.main) {
        output.push(<il-menu-main data={this.data.menu.main}></il-menu-main>);
      }
      this.data.data.map(item => {
        output.push(this.renderItems(item));
      });
      if (this.data.menu && this.data.menu.footer) {
        output.push(<il-menu-footer data={this.data.menu.footer}></il-menu-footer>);
      }
      return (
        <Host>
          {output.map(out => {
            return out;
          })}
        </Host>
      );
    }
  }