Persist column settings across plugins

[ada-x64]

First step in fixing https://github.com/finos/perspective/issues/1610

Based on #2443

This PR moves the datagrid's integer and floating point precision out of the plugin settings and into a new perspective-viewer config field called column_config. This is communicated to the plugin by passing the viewer config to the plugin directly on restore.

CHANGES:

• Create a column style control abstraction and integrate a precision control for numeric types.
• Add column_config to the viewer API.
• Update plugin.restore to accept both the plugin config and the total viewer config, allowing it to access column_config.
• Integrate the precision control for Datagrid.
• Add DEFAULT_CONFIG to perspective-viewer exports for use in tests.

TODO:

• Integrate the new config settings in the datagrid.
• Write migrations.
• Write tests.
• Future PR: Use this to integrate X/Y Scatter Plot axis labelling with column_config precision status.

[ada-x64]

The approach in this PR is not going to work. We need to overhaul a few things. Superseded by https://github.com/finos/perspective/pull/2508