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ARROW-14062: [Format] Initial arrow-internal specification of compute IR

See also #10856

Differing design decisions from the above:
- Don't special case for any `Expression`s or `Relation`s. All array functions and relations are identified by name, which may include a namespace for differentiating between extenders.
- Freely extensible without recompilation of flatbuffers (with the cost of being a less "pure" flatbuffers format since bytes blobs are used liberally).
- The root type is a Plan rather than a Relation- instead of expressing a value it is a specification of a side effect which includes the destination for output rows.

Closes #10934 from bkietz/compute-ir

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Phillip Cloud <417981+cpcloud@users.noreply.github.com>
Signed-off-by: Phillip Cloud <417981+cpcloud@users.noreply.github.com>