feat: Make most Feature fields optional

[rolodato]

Currently, defining default flags requires passing a list of Flags. Each flag contains a Feature, which has many unnecessary fields when it comes to defining default flags:

id
createdDate
description
type
defaultEnabled
initialValue

This PR makes all of these fields optional, so that default flags can be defined like this:

val defaultFlags = listOf(
    Flag(
        feature = Feature(
            name = "Flag 1",
        ),
        enabled = true,
        featureStateValue = "value1"
    ),
    Flag(
        feature = Feature(
            name = "Flag 2",
        ),
        enabled = true,
        featureStateValue = "value2"
    ),
)

instead of:

val defaultFlags = listOf(
    Flag(
        feature = Feature(
            id = 345345L,
            name = "Flag 1",
            createdDate = "2023‐07‐07T09:07:16Z",
            description = "Flag 1 description",
            type = "CONFIG",
            defaultEnabled = true,
            initialValue = "true"
        ), enabled = true, featureStateValue = "value1"
    ),
    Flag(
        feature = Feature(
            id = 34345L,
            name = "Flag 2",
            createdDate = "2023‐07‐07T09:07:16Z",
            description = "Flag 2 description",
            type = "CONFIG",
            defaultEnabled = true,
            initialValue = "true"
        ), enabled = true, featureStateValue = "value2"
    ),
)