[keyserver] Introduce sendMessagesOnAccountCreation

Summary:
This diff does two things:

1. Rename processAccountCreationCommon to sendMessagesOnAccountCreation to more accurately reflect its newly reduced scope
2. Update it to return the list of rawMessageInfos so that it can be used from createAccount (legacy registration endpoint)

I decided to do this while working on ENG-8101, but it doesn't directly address that issue.

Test Plan:

1. I tested account creation via keyserver auth in a multi-keyserver environment
2. I tested legacy account creation in a legacy environment

Reviewers: inka

Reviewed By: inka

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12038