Add pre-specified features for `observation.value_as_number`

[schuemie]

Right now, the pre-specified features include features for measurement.value_as_number, capturing the last recorded numeric value for a measurement_concept_id - unit_concept_id combination. For the observation table there are no equivalent analyses, even though it too has value_as_number and unit_concept_id fields.

There is no real reason why those pre-specified features aren't currently included, and it will be important if people have patient-reported outcomes with numeric values.

Adding them would mean

• Templating MeasurementValue.sql a bit more, so it can be used on both tables.
• Updating the PrespecAnalyses.csv and PrespecTemporalAnalyses.csv
• Updating createCovariateSettings() and createTemporalCovariateSettings()

Like their measurement counterparts, these features would be off by default.

If people are OK with this, I'm happy to prepare a PR.