packages - Add witnesses element for Congressional Hearings

[jonquandt]

For CHRG granules, we will be adding a witnesses element to display this information, similar to what we currently display in MODS. For example, here is what the field would look like for https://api.govinfo.gov/packages/CHRG-117hhrg44383/granules/CHRG-117hhrg44383/summary?api_key=DEMO_KEY

"witnesses": [
        "Joseph Aldy, Professor of the Practice of Public Policy, Harvard University",
        "Peter Erickson, Climate Policy Program Director, Stockholm Environmental Institute",
        "Tara Houska, Founder, Giniw Collective",
        "Jill Antares Hunkler, Seventh-Generation Ohio Valley Resident",
        "Greta Thunberg, Climate Activist",
        "Frank J. Macchiarola, Senior Vice President, Policy American Petroleum Institute"
    ]

The equivalent information in the MODS looks like:

<witness>Joseph Aldy, Professor of the Practice of Public Policy, Harvard University</witness>
<witness>Peter Erickson, Climate Policy Program Director, Stockholm Environmental Institute</witness>
<witness>Tara Houska, Founder, Giniw Collective</witness>
<witness>Jill Antares Hunkler, Seventh-Generation Ohio Valley Resident</witness>
<witness>Greta Thunberg, Climate Activist</witness>
<witness>Frank J. Macchiarola, Senior Vice President, Policy American Petroleum Institute</witness>