feat: allow adding second transport

[link2xt]

Tracking issue: #7357

Based on (already merged) #7424, #7413, #7406, #7392

Adding second transport is only allowed with bcc_self disabled for now, synchronization of transports will be a separate PR. This PR is fine for adding multiple transports on a single-device setup already and switching between them.

Connectivity view will display something, but extending it for multi-transport is out of scope for this PR, this is in the tracking issue.

• move resync_request from Context to Imap (already merged in refactor(imap): move resync request from Context to Imap #7406)
• add transport_id column to imap table
• add transport_id column to imap_sync table
• set mvbox_move to 0 when the first transport is created, even if it is not chatmail
• do not allow to create a second transport if mvbox_move or only_fetch_mvbox is enabled
• do not allow to enable mvbox_move or only_fetch_mvbox if more than one transport is configured
• test that current ConfiguredAddr transport cannot be deleted
• do not allow to set ConfiguredAddr to the address that does not belong to any transport
• run IMAP connections for all transports at once when I/O is started. For SMTP we only run selected transport, this works already
• test that vCards contain the current primary address
• Fix migration 73 to use direct SQL queries
• restart I/O automatically when configured_addr is set
• delete corresponding imap and imap_sync entries when the transport is deleted
• fix flaky legacy examples/test_examples.py::test_echo_quit_plugin (disabled it, want to finish this PR instead of debugging legacy CFFI test)
• do not create imap_sync entries for transport_id 0 during configuration
• clean up SMTP queue when configured_addr is changed or keep multiple SMTP connections until SMTP queue for non-primary transports is empty