[Expressions] datatable utility functions

[ppisljar]

In expressions Datatable is the common data format that as many functions as possible should share. Each DatatableColumn has a meta field, which contains additional information about the column such as field, type and format.

We want to keep the structure of meta field private (not part of our public API) and instead provide utility functions that extract information from DatatableColumnMeta instead.

This will allow us to do changes to the DatatableColumnMeta without affecting downstream consumers and also make usage easier and clearer.

The basic list of functions:

• getField(column: DatatableColumn): IndexPatternField | undefined retrieves index pattern field instance if available
• getFieldFormat(column: DatatableColumn): FieldFormat retrieves field formatter instance for specified column
• getAggConfig(column: DatatableColumn): AggConfig | undefined retrieves AggConfig instance for specified column
• getInterval(column: DatatableColumn): string | undefined retrieves interval used for the column (if it's a date histogram) - resolves auto to the actual interval
• setFieldFormat sets a new format for column
• clearField removes column association with field
• clearFieldFormat removes formatting meta from column

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)