Add support for ContactsContract.Groups.SOURCE_ID

[vestrel00]

Problem

Similar to #300, this library currently does not support reading and writing the value of Group's ContactsContract.SyncColumns.SOURCE_ID, which is a critical component to sync adapter operations.

Solution

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

Although this library is focused on supporting application use cases instead of sync adapter use cases, this field is pretty simple to add and may be used by apps for read operations/queries. It's not that out-of-place 🤔

• Regression check GroupsInsert API (new Groups with null source ID but non-null Account should have a non-null source ID automatically assigned to it post-insert assuming sync settings and network connection are enabled).
• Check that the source ID is settable using the GroupsInsert API.
• Regression check GroupsUpdate API (source ID should remain unchanged between updates if the user did not change them).
• Check that the source ID is modifiable using the GroupsUpdate API.
• Check GroupsDelete API (delete by matching source ID using WHERE clause).
• Check that GroupsQuery API (find by matching source ID using WHERE clause).
• Update all in-code documentation, where necessary.
• Update docs, where necessary.
• Update gh-pages branch, if necessary.

[vestrel00]

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This is now release in 0.3.1!
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