Add support for `ContactsContract.Groups.DIRTY`

[vestrel00]

Problem

Similar to #354, this library currently does not support reading and writing the value of Group's ContactsContract.SyncColumns.DIRTY, which is a critical component to sync adapter operations.

Solution

Add dirty field to GroupEntity and support CRUD operations for it.

• Check behavior in GroupsInsert
• Check behavior in GroupsUpdate
• Check behavior in GroupsDelete (e.g. delete by matching dirty using WHERE clause)
• Check behavior in GroupsQuery
• Update all in-code documentation
• Update docs, where necessary
• Update gh-pages branch