fhir-search parser cannot parse datetime fractionalSeconds with more than 6 digits of precision

[michaelwschroeder]

Describe the bug
When attempting to search date fields using a dateTime value with fractional seconds of more than 6 digits of precision, an error is returned: Invalid Date Time Format found please use 'yyyy-mm-ddThh:mm:ss[Z|(+|-)hh:mm].'

To Reproduce
Steps to reproduce the behavior:

1. Attempt to search a date field using a value with more than 6 digits of precision for fractional seconds: https://localhost:9443/fhir-server/api/v4/DocumentReference?date=2020-08-06T15:36:41.948000000Z

Expected behavior
In order to be consistent with parsing of dateTime and instant fields on object creation (see issue #1446), up to 9 digits of precision should be allowed for fractional seconds on input, and should be truncated to 6 digits of precision when executing the search request.

Additional context
Add any other context about the problem here.

[lmsurpre]

I reviewed the tests included in the PR. LGTM.