This is Part 2 in a stacked PR set that delivers aggregate root field
support.
* Part 1: OpenDD: hasura/v3-engine#683
* Part 3: GraphQL API: hasura/v3-engine#685

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

## Description
This PR implements the metadata resolve phase of the engine and adds
support for resolving `AggregateExpression`s and validates their use
when linked to a `Model`.

The bulk of the changes can be found in:
* `crates/metadata-resolve/src/stages/aggregates/*` - This is where the
`AggregateExpression`s are resolved
* `crates/metadata-resolve/src/stages/models/mod.rs` - This is where we
validate the `AggregateExpression` specified for use in the model is
actually compatible with the model and its data connector

The `ndc-spec` version used has been lifted to the latest version that
adds support for aggregates over nested objects
(hasura/ndc-spec#144). This necessitated changes
in the Custom Connector, but actual functionality to implement
aggregation over nested objects is implemented in Part 3.

There are also some changes in
`crates/metadata-resolve/src/types/subgraph.rs` where the `Display`
trait for the various `Qualified<T>`, `QualifiedTypeReference`, etc
types has been reworked so that they print more cleanly, with the
subgraph being put outside the type syntax, and array types getting
formatted correctly. For example, previous an array of Varchars would
have printed as `Varchar (in subgraph default)!`, now it properly
formats as `[Varchar!]! (in subgraph default)`.
This was necessary to make useful error messages using these types.

A tonne of tests have been added in
`crates/engine/tests/validate_metadata_artifacts/aggregate_expressions`
to test every error condition of the metadata resolve process.

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

V3_GIT_ORIGIN_REV_ID: ffd859127a3f1560707f06ef01906c9d1b183d31