feat: add missing rels to rel message

[westonpace]

The ReferenceRel, WriteRel, and DdlRel were defined in algebra.proto but not part
of message Rel which meant they were unusable. This PR adds those back. It is
inspired by #288 but more targeted in scope. One change from that original PR which I
also kept was replacing the word tuple with record in the documentation for consistency.

This is not to imply that ReferenceRel, WriteRel, or DdlRel are "complete" or "stable"
in any way. I feel these relations are still quite ill defined. However, my hope is that by
making them usable we can inspire further change to them.

BREAKING CHANGE: The enum WriteRel::OutputMode had an option change from
OUTPUT_MODE_MODIFIED_TUPLES to OUTPUT_MODE_MODIFIED_RECORDS
BREAKING CHANGE: The message AggregateFunction.ReferenceRel has moved
to ReferenceRel.

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[westonpace]

This is failing the breaking change check for these two reasons:

 proto/substrait/algebra.proto:560:36:Enum value "2" on enum "OutputMode" changed name from "OUTPUT_MODE_MODIFIED_TUPLES" to "OUTPUT_MODE_MODIFIED_RECORDS".
proto/substrait/algebra.proto:1354:1:Previously present message "AggregateFunction.ReferenceRel" was deleted from file.

Since these were technically unreachable features (from Plan or ExtendedExpression) I am hesitant to label this an actual breaking change. I'm willing to go ahead and do so though if others feel it is needed.

[EpsilonPrime]

This is failing the breaking change check for these two reasons:

 proto/substrait/algebra.proto:560:36:Enum value "2" on enum "OutputMode" changed name from "OUTPUT_MODE_MODIFIED_TUPLES" to "OUTPUT_MODE_MODIFIED_RECORDS".
proto/substrait/algebra.proto:1354:1:Previously present message "AggregateFunction.ReferenceRel" was deleted from file.

Since these were technically unreachable features (from Plan or ExtendedExpression) I am hesitant to label this an actual breaking change. I'm willing to go ahead and do so though if others feel it is needed.

I'd be inclined to just say that it will break to compilation for anyone using them directly.

[westonpace]

I'd be inclined to just say that it will break to compilation for anyone using them directly.

Fair enough.

[EpsilonPrime]

A good step forward, thanks!

[jacques-n]

LGTM