Support using "direct" as the message "type" for private messages.

[laurynmm]

As follow-up to zulip/zulip#25154 (which fixed zulip/zulip#24960), we should:

• Update sending messages (POST /messages) to support using "direct" as the message type for direct/private messages.
• Auto-translate "direct" to "private" for servers that it knows via the the Client initialization check on the API feature level don't support it; "direct" was introduced at API feature level 174.

See #api design > message type parameter, #24960 for relevant context and discussion.