Use DateTimeFormatter for javatime codecs

[johnspade]

Javatime codecs should serialize dates and times with formats that are used to parse them.
Tapir uses OffsetDateTime's .toString to encode Instants, but you can't parse back some of its outputs:

import java.time.Instant
import sttp.tapir.Codec.instant

val epoch = instant.encode(Instant.EPOCH)
// epoch: String = 1970-01-01T00:00Z
Instant.parse(epoch)
// java.time.format.DateTimeParseException: Text '1970-01-01T00:00Z' could not be parsed at index 16

I think it's better to be more explicit about formats and not to rely on .toString for javatime codecs.

[adamw]

what about tests for these types? would be good if they are still covered

[johnspade]

Added missing tests

[adamw]

Released in 0.17.0-M2, thanks!