JITMs: add filter to insert affiliate codes in upgrade notices

It would be nice if WP developers and agencies were allowed to use their affiliate code in JITMs, via a filter, so they can configure the JITMs for the sites they have installed and manage for their clients.

Affiliate URLs look like this: `https://refer.wordpress.com/r/XXXX/jetpack/` and redirect to pages like this one: `https://jetpack.com/pricing/?affiliate=XXXX`

No files or tests are named. Start by locating the JITM upgrade-notice entry point and tracing how its affiliate URL is built; then define the filter's input and verify that configured affiliate codes produce the documented redirect URL without changing notices when unset.

Written by the indexing model from the issue text.