Add communication_disabled_until to GuildMemberParams

[0xAozora]

This allows to timeout an user.

Closes #1213

[FedorLap2006]

We usually use time.Time for timestamps.
Additionally this field must be both omittable and nullable (to unmute someone, a null value must be set).
There are two ways to implement that:

• Custom MarshalJSON with *time.Time
• NullTime type

NullTime might seem easier and more understandable, but it requires a lot of consideration, as we try to avoid helper types. MarshalJSON on other hand is easy to implement, and we've done that a few times, but might be weird to use for the user, as they would need to specify time.Time{} (zero'ed time) to unmute someone.

[FedorLap2006]

Since GuildMemberParams had other unimplemented params and the communication_disabled_until needs custom marshalling for the null, I've decided to implement it all together in #1226. Therefore I'm going to close this PR.

[0xAozora]

Cool!