Add a context manager for imposing a solar-radius value

[ayshih]

This PR adds a context manager as an option for dealing with the situation of inconsistent values for the solar radius. This context manager makes it so that the rsun frame attribute for any relevant coordinate frame reports as the specified value, with 695.7 Mm as the default:

>>> import astropy.units as u

>>> import sunpy.map
>>> from sunpy.coordinates import impose_solar_radius
>>> from sunpy.data.sample import AIA_171_IMAGE

>>> aiamap = sunpy.map.Map(AIA_171_IMAGE)
>>> print(aiamap.observer_coordinate)
<SkyCoord (HeliographicStonyhurst: obstime=2011-06-07T06:33:02.770, rsun=696000.0 km): (lon, lat, radius) in (deg, deg, m)
    (-0.00406308, 0.04787238, 1.51846026e+11)>

>>> with impose_solar_radius():  # defaults to the IAU value of 695.7 Mm
...    print(aiamap.observer_coordinate)
<SkyCoord (HeliographicStonyhurst: obstime=2011-06-07T06:33:02.770, rsun=695700.0 km): (lon, lat, radius) in (deg, deg, m)
    (-0.00406308, 0.04787238, 1.51846026e+11)>

If you perform a reprojection under this context manager, there will no longer be missing data due to a rsun mismatch.

[github-actions]

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