[native] Add avatar column to clientDB threads table

Summary:
We are adding an avatar column to the threads table in the clientDB and including an avatar field in the Thread struct. At the moment, the value of this field will always be NULL until we make some necessary changes. These changes involve modifying:

1. The translation functions that convert between ClientDBThreadInfo and ThreadInfo.
2. The C++ code responsible for constructing JSI ClientDBThreadInfo objects from the threads table rows retrieved from the database on the "JS side."

Once these changes have been made, we will be able to update the avatar field in the threads table and properly persist data in the new avatar column.

Test Plan:

1. Verify that the migration was successful:

Before:

After:

2. Ensure that freshly created clientDB includes avatar column in the threads table:

Reviewers: ashoat, ginsu, kamil

Reviewed By: ashoat, kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7257