Fix handling of creator client in MLS conversations

[pcapriotti]

When an MLS conversation is created, it implicitly contains the creator client as a member. This was not reflected in the database. This PR fixes the problem as follows:

• it requires a creator_client field when a conversation is created, and uses it to record the creator client as a member of the conversation
• it associates the key package ref of the creator to that client when the first commit is processed.

This required changing the conversation creation endpoint POST /conversations. The change is only visible in v2.

Checklist

• 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.
• changelog.d contains the following bits of information (details):
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  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[CLAassistant]

All committers have signed the CLA.

[flokli]

This accidentially reverted the submodule bump in nginx-module-vts.

[flokli]

PR to undo in #2566.