Allow passing entire chart definition to $chart['#chart_definition']

Problem/Motivation

The $chart element has a temporary '#chart_definition' property used to hold the specific configuration options generated by the active charting library plugin (e.g., Google Charts, Highcharts). This definition array is then JSON-encoded and passed to the frontend as a data-chart HTML attribute, which the client-side JavaScript reads to actually draw the chart.

Currently, we are a bit restrictive and have the module populate that property, which ensures that all the necessary elements are provided, but it also prevents a user from using a chart type (for example) that's not supported by the rest of the Charts module code.

Proposed resolution

If we refactor how we handle that property, we can enable a user to pass a chart definition generated outside of the Charts module, opening the module up to many more chart types being supported (and other properties not currently supported by the Charts module).

Remaining tasks

1) Refactor
2) Test, including ensuring that the property value remains sanitized.

User interface changes

None.

API changes

TBD.