feat: Instant support

[eranl]

Add support for java.time.Instant as a timestamp type.
This is a port of googleapis/java-firestore#1586.

[eranl]

Following up. @tom-andersen, I just realized you requested that I tag you here.

[eranl]

@tom-andersen, the latest test failures seem to be unrelated to my change, and this branch is fully caught-up.

[tom-andersen]

Thanks! I will copy the branch over to make the CI run properly.

I will also need to understand how this is backward compatible.

[eranl]

Thanks! I will copy the branch over to make the CI run properly.

Thanks.

I will also need to understand how this is backward compatible.

The same way googleapis/java-firestore#1586 was.

[dconeybe]

Consider adding support for kotlinx.datetime.Instant as well.

https://kotlinlang.org/api/kotlinx-datetime/kotlinx-datetime/kotlinx.datetime/-instant/

[eranl]

Consider adding support for kotlinx.datetime.Instant as well.

Done. Also added tests.

[dconeybe]

Please change this dependency to "compileOnly" so we don't unnecessarily pull in a dependency on the kotlinx.datetime library if it's not used by the downstream dependent.

[eranl]

Done.