datetime, lua: strptime-like parse format

To parse date/time strings using format string we use
`strptime()` implementation from FreeBSD, which is
modified to use our `struct datetime` data structure.

List of supported format has been extended to include
`%f` which is flag used whenever you need to process
nanoseconds part of datetime value.

```
tarantool> T = date.parse('Thu Jan  1 03:00:00 1970', {format = '%c'})

tarantool> T
- 1970-01-01T03:00:00Z

tarantool> T = date.parse('12/31/2020', {format = '%m/%d/%y'})

tarantool> T
- 2020-12-31T00:00:00Z

tarantool> T = date.parse('1970-01-01T03:00:00.125000000+0300',
                          {format = '%FT%T.%f%z'})

tarantool> T
- 1970-01-01T03:00:00.125+0300
```

Part of #6731

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