The time_format struct tag works for form/query binding but is completely
ignored when binding via JSON (ShouldBindJSON). This commit adds support
for time_format, time_utc, and time_location tags in the JSON binding path.
When a struct has time.Time fields with time_format tags, the JSON decoder
falls back to mappingByPtr with a jsonSource setter that uses setTimeField
to parse the string value with the custom format. For structs without
time_format tags, the original fast path (direct JSON unmarshal) is used,
preserving full backward compatibility.
Fixes#2170
04a7275c71