Automatic migration support while adding columns + pretty-printing

[kazimuth]

• BitCraft may need to be able to add columns to tables
• User should get a report on what changes will happen during an auto-migration BEFORE the migration happens
• If the auto-migration will require clients to go down and update, there should be big bold red warnings

[kazimuth]

See https://github.com/clockworklabs/SpacetimeDBPrivate/pull/1849

TODOs:

• Implement adding columns in auto-migrations in the schema crate
• Implement default column values in the module library macros
  • Rust
  • C#
• Implement a pre_publish API endpoint that gets a printable version of an auto migration plan from the database.
  • Decide how exactly the flow should work between pre_publish and publish
  • Note: see Pretty print auto-migration plans #2065 for a partial implementation of plan printing. The plumbing here was wrong, because it did the publishing in a single HTTP call, rather than two. This meant that the user only got the summary after the migration had already been applied. We need two separate calls to print the summary before the plan is applied, and get user approval.
  • If the plan includes a DisconnectAllUsers step, we need to print a VERY LOUD WARNING in the plan.
• Implement the actual migration in core::db::update. This will need to
  • Read the table into memory, update it, write it back
  • Update system tables
  • Update any other metadata that might be floating around

[egormanga]

How this would work with WAL, e.g. history feature?

[Shubham8287]

Closued with #2962