Add serde helpers for base64 encoding/decoding

[jhendrixMSFT]

To be used in serde "with" derive macros for struct fields that contain base64 data.

pub struct Base64BytesProperty {
    #[serde(
        deserialize_with = "base64::deserialize",
        skip_serializing_if = "Option::is_none",
        serialize_with = "base64::serialize"
    )]
    pub value: Option<Vec<u8>>,
}

[heaths]

Nothing blocking, but please add some comments (all new code should; I'll do a pass on old stuff before we GA if not sooner) and maybe a round-trip test.