Allow RelBuilder to create RelNode with convention and use it for trait convert

1. Provide Convention.transformRelBuilder() to transform an existing RelBuilder into one with specific convention.
2. RelBuilder provides withRelFactories() method to allow caller swap the underlying RelFactories and create a new builder.
3. Use the new interface in RelCollationTraitDef for converting into RelCollation traits

We can avoid ~1/3 of total rule firings in a N way join case with this change.