Add `client` to `TagKind`

[dluvian]

Describe the enhancement

Add client to TagKind enum.

Use case

I want to get the value of a client tag of an event.

It would be nice if I could call event.tags().find { it.kind() == TagKind.Client } instead of using asVec() to get the value as I am doing now.

Additional context

https://github.com/nostr-protocol/nips/blob/master/89.md#client-tag