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

[link2xt]

This is now ready for review.

It is a minimal support for multi-transport, allowing to add transports and switch between them while being connected to all of them over IMAP. It does not support synchronization, so adding transports should only be allowed when multi-device is not enabled ("bcc_self" is false).

Most important to review is the SQL migration, everything else can be changed later.