wp.hooks.addFilter(
    'blocks.registerBlockType',
    'myPlugin/quoteButtonAttributes',
    settings => {
        if(settings.name === 'core/pullquote') {
            settings.attributes = {
                ...settings.attributes,
                myPluginShowButton: {
                    type: 'boolean',
                    default: false,
                },
                myPluginButtonContent: {},
            };
        }
        return settings;
    }
);

wp.hooks.addFilter(
    'editor.BlockEdit',
    'myPlugin/quoteButtonEdit',
    wp.compose.createHigherOrderComponent(
        BlockEdit => props => {
            if(props.name === 'core/pullquote') {
                const {attributes, setAttributes} = props;
                const {myPluginShowButton, myPluginButtonContent} = attributes;
                return (
                    <wp.element.Fragment>
                        <BlockEdit {...props} />
                        {myPluginShowButton && (
                            <button>
                                <wp.blockEditor.RichText
                                    tagName='div'
                                    placeholder='Click here'
                                    value={myPluginButtonContent}
                                    onChange={value => setAttributes({myPluginButtonContent: value})}
                                />
                            </button>
                        )}
                        <wp.blockEditor.InspectorControls>
                            <wp.components.PanelBody title='Button options'>
                                <wp.components.ToggleControl
                                    label='Show button'
                                    checked={myPluginShowButton}
                                    onChange={() => setAttributes({myPluginShowButton: !myPluginShowButton})}
                                />
                            </wp.components.PanelBody>
                        </wp.blockEditor.InspectorControls>
                    </wp.element.Fragment>
                );
            }
            return <BlockEdit {...props} />;
        },
        'withMyPluginQuoteButtonEdit'
    )
);

wp.hooks.addFilter(
    'blocks.getSaveElement',
    'myPlugin/quoteButtonSave',
    (element, block, attributes) => {
        if(block.name === 'core/pullquote') {
            const {myPluginShowButton, myPluginButtonContent} = attributes;
            if(myPluginShowButton && myPluginButtonContent) {
                return (
                    <wp.element.Fragment>
                        {element}
                        <button>
                            <wp.blockEditor.RichText.Content value={myPluginButtonContent} />
                        </button>
                    </wp.element.Fragment>
                );
            }
        }
        return element;
    }
);