This is Part 1 in a stacked PR set that delivers aggregate root field
support.
* Part 2: Metadata Resolve: hasura/v3-engine#684
* Part 3: GraphQL API: hasura/v3-engine#685

JIRA: [V3ENGINE-159](https://hasurahq.atlassian.net/browse/V3ENGINE-159)

## Description
This PR is the first cut in implementing aggregates functionality for
v3, as defined in the [Aggregates and Grouping
RFC](https://github.com/hasura/v3-engine/blob/main/rfcs/aggregations.md).

This PR modifies the Open DD schema and adds the `AggregateExpression`
metadata object. It also modifies the `Model` and `GraphqlConfig` types
and adds new fields that are required to link in aggregates
functionality. For more information, please see the below RFC links.

This PR implements only the minimum properties required to get a
aggregates root fields in the GraphQL API. There is more defined in the
RFC that will be implemented in later iterations.

RFC links:
*
[AggregateExpression](https://github.com/hasura/v3-engine/blob/main/rfcs/aggregations.md#aggregateexpression-new)
*
[Model](https://github.com/hasura/v3-engine/blob/main/rfcs/aggregations.md#model--object-type)
*
[GraphqlConfig](https://github.com/hasura/v3-engine/blob/main/rfcs/aggregations.md#graphql-config)

## Changelog

### Product

_(Select all products this will be available in)_

- [x] community-edition
- [x] cloud
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### Type

- [ ] highlight
- [x] enhancement
- [ ] bugfix
- [ ] behaviour-change
- [ ] performance-enhancement
- [ ] security-fix
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### Changelog entry

Support for aggregate GraphQL root fields via the new
AggregateExpression OpenDD type and its use on the Model type

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[V3ENGINE-159]:
https://hasurahq.atlassian.net/browse/V3ENGINE-159?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

V3_GIT_ORIGIN_REV_ID: 6969ee73036a011e3026b58545c0286a65749067