Allow modification of exposure window without redefining the entire exposure signal

[danielkberry]

The exposure window is, by default, the enrollment period. Sometimes this should be changed to another period. It should be easy for users to change the time window we look for exposures, without manually writing a new exposure signal.

Ideally, the following should work:

[experiment.exposure_signal]
window_start = 0
window_start = 14

Currently, this generates missing parameter errors. IMO, we should allow users to specify either 1) only these 2 fields or 2) all fields. In the case of 1, metric-config-parser should layer these fields on top of (and override) the defaults.

┆Issue is synchronized with this Jira Task