Add savepoint support

[roji]

• Add APIs for supporting transaction savepoints.
• Support is implemented at the EF level only, no System.Data support yet (this should come soon).
• Use savepoints in the update pipeline when a user-managed transaction is used, to roll back to before SaveChanges in case of exception.

Some notes:

• See SaveChanges_uses_explicit_transaction_with_failure_behavior for how savepoints help with SaveChanges failures.
• Another good example is Database_concurrency_token_value_is_discarded_for_non_conflicting_entities, which isn't needed anymore because the entire SaveChanges is rolled back if any conflict occurs. I wonder if we should still have a test for this - this may require an escape hatch flag that allows running without savepoints even if the provider supports them (but I'm not sure where someone would use that).
• This currently implements everything in EF Core, without any System.Data support (Expose transaction savepoints in ADO.NET runtime#33397). When that makes it in, I'll replace the default implementation in RelationalTransaction to delegate to that.
• We should have logging for savepoints like other things, but we can wait for the System.Data implementation to put them into the default implementation. Same for interceptors.
• AFAICT ambient transactions (System.Transactions) don't support savepoints. You can have multiple nested scopes, but that's just a vote for committing the top-level scope.

Part of #20176

[roji]

/cc @ajcvickers