[json_codable] Support for specifying how DateTime format will be encoded out of the box

[alestiago]

As a developer using json_codable I would like to have the option to specify what DateTime format should be used for encoding.

This issue was derived from this @jakemac53 Gerrit review comment:

Eventually we probably need to have some sort of system for custom serialization of specified types or fields, but for now I don't really see any reason not to use ISO860 strings other than efficiency, but it is probably a negligible thing.

Additional context:

• https://dart-review.googlesource.com/c/sdk/+/394401
• [json] add datetime support #57063 (comment)
• [json] add datetime support #57063

[dart-github-bot]

Summary: Users want json_codable to support custom DateTime encoding formats, avoiding the default ISO8601 for potential efficiency gains.

[jakemac53]

We definitely do want to enable custom encoding of all supported types. This is going to be a ways off most likely though, we are in a holding pattern right now while some of the underlying macro APIs are worked out.