API review changes

[ajcvickers]

Part of #33220

• Replace IUpdateEntry.GetOriginalOrCurrentValue with HasOriginalValue
• Review whether we really need MigrationsSqlGenerator.SequenceOptions overload with forAlter. Move logic to other methods otherwise.
• Remove IReadOnlySequence.IsCached. CacheSize of 0 or 1, indicates no caching, all other non-null values indicate caching

[ajcvickers]

I think we're spending way too much time on this. I'll give it one more day to get to some agreement on the behavior we want, otherwise I'll back this change out and we can put it back on the backlog.

[ajcvickers]

Superseded by #34240