[FEATURE] Codes for categorical data

[dinya]

Problem description

Currently the only way to represent categorical data in CDS (on the JS side) are strings.

If the data source is large enough, it will take up a lot of memory on the client side.

Feature description

It is proposed to use int values to encode categories and mapping {code:"string"} like Pandas does.

Potential alternatives

Additional information

After https://discourse.bokeh.org/t/codes-for-categorical-data/11410/2