[native] Support avatar uploads to blob service

Summary:
This diff enables avatar uploads to blob service on native. Modified the hook to conditionally use function that uploads avatar to blob service instead of keyserver. This is very similar to what we currently do with multimedia messages.

Depends on D9668

Test Plan:
Similiar to D9170 test plan, but on native:

Tested on both user and thread avatars.
With the flag set to true, changed the avatar to an image and used TablePlus to verify MariaDB contents (this tests changes to D9168). Also, the avatar was then displayed correctly (which is more deeply tested in D9624 and D9668)

Reviewers: atul, ginsu

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9669