feat: support shared content (#269)

This PR introduces a pre-processing step scanning the gql schemas from
Sanity for directives that indicate a field or type as a cross dataset
reference. When these directives are found, the corresponding fields are
manipulated to be treated as normal references with the expectation that
the user will add another source config for the target dataset in the
Gatsby app configuration. Through Gatsby schema composition the types
should then match up, and the node resolution should also pick out the
target documents.

The main implementation to review is found in 

`packages/gatsby-source-sanity/src/util/mapCrossDatasetReferences.ts`

End to end tests de-referencing both normal and cross dataset references
from actual Sanity datasets are included in the `cdrs` Gatsby app in
`examples`.

To run tests, first make sure the test app is running

```bash
cd examples/cdrs && rm -rf .cache public && npm run develop
```

Then run the test suite in that folder
```bash
cd examples/cdrs && npm test
```

---------

Co-authored-by: Binoy Patel <me@binoy.io>

Author: runeb

examples/cdrs/.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# Runtime data
+pids
+*.pid
+*.seed
+*.pid.lock
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# nyc test coverage
+.nyc_output
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# Bower dependency directory (https://bower.io/)
+bower_components
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directories
+node_modules/
+jspm_packages/
+
+# Typescript v1 declaration files
+typings/
+
+# Optional npm cache directory
+.npm
+
+# Optional eslint cache
+.eslintcache
+
+# Optional REPL history
+.node_repl_history
+
+# Output of 'npm pack'
+*.tgz
+
+# dotenv environment variable files
+.env*
+
+# gatsby files
+.cache/
+public
+
+# Mac files
+.DS_Store
+
+# Yarn
+yarn-error.log
+.pnp/
+.pnp.js
+# Yarn Integrity file
+.yarn-integrity

examples/cdrs/.prettierignore

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+.cache
+package.json
+package-lock.json
+public

examples/cdrs/.prettierrc

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+{
+  "arrowParens": "avoid",
+  "semi": false
+}

examples/cdrs/README.md

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+This is a test Gatsby app for end-to-end testing the gatsby-source-sanity plugin with Sanity Content Lake data and schema
+
+# Testing
+Manually run the application
+
+```bash
+rm -rf .cache public && npm run develop
+```
+
+Run the test suite
+
+```bash
+npm test
+```

examples/cdrs/gatsby-config.js

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+/**
+ * Configure your Gatsby site with this file.
+ *
+ * See: https://www.gatsbyjs.com/docs/reference/config-files/gatsby-config/
+ */
+
+const path = require("path")
+const pluginPath = path.resolve(
+  __dirname,
+  "../../packages/gatsby-source-sanity"
+)
+
+/**
+ * @type {import('gatsby').GatsbyConfig}
+ */
+module.exports = {
+  siteMetadata: {
+    title: `Gatsby Default Starter`,
+    description: `Kick off your next, great Gatsby project with this default starter. This barebones starter ships with the main Gatsby configuration files you might need.`,
+    author: `@gatsbyjs`,
+    siteUrl: `https://gatsbystarterdefaultsource.gatsbyjs.io/`,
+  },
+  plugins: [
+    {
+      resolve: pluginPath,
+      options: {
+        apiHost: "https://api.sanity.work",
+        projectId: "rz9j51w2",
+        dataset: "production",
+      },
+    },
+    {
+      resolve: pluginPath,
+      options: {
+        apiHost: "https://api.sanity.work",
+        projectId: "rz9j51w2",
+        dataset: "shared",
+      },
+    },
+    `gatsby-plugin-image`,
+  ],
+}