Add MLS feature config

[smatting]

https://wearezeta.atlassian.net/browse/FS-667

This PR also updates the checklist for adding new features

Checklist

• manually checked stern
• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, or feature configs have changed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If new config options introduced: recommended measures to be taken by on-premise instance operators.

[pcapriotti]

Looks good, except that the default protocol settings isn't honoured by createGroupConversation. Would it be hard to make it query the value of the setting, and decide the protocol accordingly? That would require making the protocol field of type Maybe ProtocolTag in NewConv as well. I'm not sure if this is necessary.

Alternatively, we could simply hardcode the default and set it to proteus.

[smatting]

Looks good, except that the default protocol settings isn't honoured by createGroupConversation. Would it be hard to make it query the value of the setting, and decide the protocol accordingly? That would require making the protocol field of type Maybe ProtocolTag in NewConv as well. I'm not sure if this is necessary.

I interpret this MLS feature config as a client configuration. So clients would should honour this config and set newConvProtocol in the NewConv accordingly

[pcapriotti]

I interpret this MLS feature config as a client configuration. So clients would should honour this config and set newConvProtocol in the NewConv accordingly

It's probably going to result in some confusion, but fair enough.

[smatting]

I've updated the docs stating these assumptions