Add --strict flag to call, getopt and to-proto subcommands

When specified, unknown and duplicate fields (in case of to-proto) and options
(in case of call and getop) are treated as errors rather than warnings.
(piqi) Add a new `json-omit-missing` field property

This optional setting controls whether to omit missing fields from JSON output
that would otherwise be represented as JSON NULL for missing optional and []
for missing repeated fields; if not specified, the default behavior is
determined by the json_omit_null_fields run-time setting which is, in turn, set
to true by default.