Add CLM protocol to get Welcome Message

[softins]

Short description of changes

Adds two CLM protocol messages: one to request the server's Welcome Message, and the other to respond with it.

CHANGELOG: Server: Add protocol for requesting a server's Welcome Message without connecting

Context: Fixes an issue?

Provides a way for a client to fetch the Welcome Message without needing to start an audio connection.
It will be very useful for Jamulus Explorer, and could be used in the future by a Jamulus client.

Does this change need documentation? What needs to be documented and how?

Not at present. It is documented in protocol.cpp. If in the future a Jamulus client uses the feature to fetch a Welcome Message, that usage would need to be documented.

Status of this Pull Request

Ready and tested.

What is missing until this pull request can be merged?

Just review

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[dingodoppelt]

Looks perfectly fine (by the code, that is. haven't tested, yet)
Could it be useful if we somehow logged requests like this one (or server features cl message)?
I just recently noticed some client was connecting to my public server every 30 seconds to do whatever. I only noticed because the connection was logged.

[ann0see]

Meaning we should probably change the client functionality in future to request the server welcome message instead of the server pushing it.

[softins]

Meaning we should probably change the client functionality in future to request the server welcome message instead of the server pushing it.

I don't think there is any point in doing that. The server still needs to be able to push chat messages to the client. The welcome message is just the first chat message to be pushed after connecting. I don't see that as a problem to be solved.

What may possibly be useful would be providing some way within the connect dialog to check the welcome message of a server before deciding whether to connect to it.