SERVER-86549 brought a substantial improvement in BSON iteration speed. We should investigate using this in SBE to have the same improvement.

At the same time, SERVER-86119 landed an improvement in BSONObj::getField() by computing the key string length while determining if the field name matches. This could be applied directly to the SBE `getField()` operation.

Note that we do not necessarily have to use BSONObj/BSONElement directly, we can just copy the pattern/idea behind it, or move the existing code to a low-level helper function.