[sankey]Update 7.15.x .

[ch-bas]

• This MR should update the Sankey Visualisation to work with Kibana's 7.15.0 version.
• A major code refactor was made in order to align the logic with the new Kibana's Plugin Architecture.
• For this version, AngularJs is still used, but a new version written in React was not release due the absence of a compatible library os d3-plugins-sankey.