Add support for "soh pp 100.20" style sms in EWS

There is no ticket, copy pasting the docs from the EWSStockReportParser class:

    EWS has a slightly different requirement for SMS
    formats, this class exists to break that functionality
    out of the default SMS handler to live in the ewsghana
    custom area.

    They send messages of the format:

        'soh nets 100.22'

    In this example, the data reflects:

        nets = product sms code
        100 = the facility stating that they have 100 nets
        20 = the facility stating that they received 20 in this period

    There is some duplication here, but it felt better to
    add duplication instead of complexity. The goal is to
    override only the couple methods that required modifications.

i agree with this sentiment. some possible alternatives:

1. Move the `EWSParser` to core (and rename to something more general) and make it a domain-level feature flag or a domain setting
2. Have a `STOCK_REPORT_PARSERS` setting that maps domains to importable class paths (kind of like `CUSTOM_MODULES` or `HQ_FIXTURE_GENERATORS`)
3. Something else clever.