[lib] Move getRolePermissionBlobs to lib

Summary:
We're going to use getRolePermissionBlobs to calculate thread permissions on the client "from first principles" (ie based on threadTypes and getRolePermissionBlobs without needing roles/permissions from the keyserver).

This diff just moves getRolePermissionBlobs to lib so we can use it from native.

Test Plan:
This should be a super straightforward drag and drop diff.

However, since it's consumed by updateRolesAndPermissionsForAllThreads I kicked off a updateRolesAndPermissionsForAllThreads migration on keyserver and made sure that roles/permissions continued to be calculated as expected... though it would be very surprising if it didn't.

Reviewers: ginsu, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7581