feat: add anchor for non-ordinal plan reference

[drin]

This adds an "anchor" to PlanRel that can be referenced from a ReferenceRel. This anchor and reference relationship provides a non-ordinal method for identifying and accessing a "subtree" or sub-graph. To maintain consistency with type_anchor and function_anchor, the new subtree_anchor attribute is uint32.

This PR leaves in the original subtree_ordinal attribute since it seems a (mildly) more performant method for referencing a subtree, but also since it is still relevant in the typical case. The new anchor attribute is an improvement for cases where multiple plans are merged and at least one already contains a ReferenceRel.

It is expected that only one of subtree_ordinal or subtree_reference will be used, however I don't see a good reason to enforce the use of only one, so I did not group the attributes in a oneof constraint.

If either of the above points ((1) keeping subtree_ordinal and (2) not enforcing a oneof constraint) are changed, then this PR would likely become a breaking change.

Also, this PR addresses #725 , but uses the name subtree_reference and subtree_anchor (instead of plan_anchor) to be consistent with the pre-existing subtree_ordinal attribute. I don't know that relations in a substrait plan are strictly trees, so I think this could be a good time to change "subtree" to "planrel" or "subgraph" or something similar.

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[tokoko]

This PR leaves in the original subtree_ordinal attribute since it seems a (mildly) more performant method for referencing a subtree

This is technically true, but on the other hand we are already doing the same thing with function extensions when the number of functions used in a typical plan most likely will be a lot bigger than the number of subtrees. I don't think it makes sense to keep the existing behavior only due to performance considerations.

It is expected that only one of subtree_ordinal or subtree_reference will be used, however I don't see a good reason to enforce the use of only one, so I did not group the attributes in a oneof constraint.

I think leaving both options especially w/o oneof will lead to a lot more confusion and possible divergence in implementations. My choice would be to deprecate subtree_ordinal and eventually get rid of it, but oneof is also a viable option.

If either of the above points ((1) keeping subtree_ordinal and (2) not enforcing a oneof constraint) are changed, then this PR would likely become a breaking change.

Pretty sure putting an existing field in a oneof is a backwards-compatible change.