Support priority for Brave Ads

[jsecretan]

Description

Certain campaigns may need to be prioritized over others for a number of reasons. If certain campaigns have commitments attached to them and are underdelivering, they may need to serve over others available. This is common practice in ad trafficking.

This is dependent on the completion of #5231. On the campaign level, there is a "priority" field with a numerical value from 1-10, 1 meaning the highest priority. For the list of available ads to serve, the native ad library should sort eligible ads ascending by this value, and select by the highest priority first.

The round robin algorithm (#5945) would also change to work first within higher priority levels before going to lower priority levels.

[tmancey]

@jsecretan Assigning to see if this issue is still relevant, thanks

[tmancey]

Blocked while deciding if this issue is still relevant