rfc: multi-column foreign key relationships

Currently relationships based on foreign key constraints would only work for single column constraints. Console works around this issue by creating `manual_configuration` in such cases. Ideally this isn't much work to support on the server and opens up possibilities for improved query generation (see https://github.com/hasura/graphql-engine/issues/5949).

## Object relationships:

- Many-To-One:

```
name: RelationshipName
table: TableName
using:
foreign_key_constraint_on: ColumnName | [ColumnName]
manual_configuration:
```

- One-To-One:
```
name: RelationshipName
table: TableName
using:
foreign_key_constraint_on:
table: TableName
# column will now be deprecated
column: ColumnName
# only one of column/columns should be present
columns: ColumnName | [ColumnName]
manual_configuration:
```

## Array Relationships:

```
name: RelationshipName
table: TableName
using:
foreign_key_constraint_on:
table: TableName
# column will now be deprecated
column: ColumnName
# only one of column/columns should be present
columns: ColumnName | [ColumnName]
manual_configuration:
```

## Potential issues

`export_metadata` will now have single element column arrays:

- Previously

```
table: article
name: author
using:
foreign_key_constraint_on: author_id
```

- After this change

```
table: article
name: author
using:
foreign_key_constraint_on: [author_id]
```

The issue names no files, tests, or entry points; start by tracing server handling of foreign_key_constraint_on metadata and existing manual_configuration relationships. Done means multi-column object and array relationships are supported, and exported metadata uses the specified column-array representation.

Written by the indexing model from the issue text.