plugin: add release datetime field to GstPluginDesc and set it if GST_PACKAGE_RELEASE_DATETIME is defined

This is a string describing a date and/or date/time in a simple subset of
the ISO-8601 format, namely either "YYYY-MM-DD" or "YYYY-MM-DDTHH:MMZ" (with
'T' the date/time separator and the 'Z' indicating UTC).

The main purpose of this field is to keep track of plugin and element versions
on an absolute timeline, so it's possible to determine which one is newer when
comparing two date time numbers. This will allow us to express 'replaces'-type
relationships betweeen plugins and element factories in future, even across
different modules and plugin merges or splits (source module version numbers
aren't particularly useful here, since they can only meaningfully be compared
within the same module). It also allows applications and libraries to reliably
check that a plugin is recent enough without making assumptions about modules
or module versions.

We use a string here to keep things simple and clear, esp. on the build system
side of things.

https://bugzilla.gnome.org/show_bug.cgi?id=623040