feat(gatsby-source-graphql) Execute functions in header. (#20731)

* feat(gatsby-source-graphql) Execute functions in headers.

Resolves issue #20690, allowing the use of async functions to define
headers for gatsby-source-graphql. To use, functions can be passed in
the headers object, i.e: headers { authorization: asyncFunc }.

* Changes whole headers parameter to accept async function.

See #20731. Updated README.md to
include new (optional) syntax.

packages/gatsby-source-graphql/README.md

@@ -42,6 +42,12 @@ module.exports = {
           // Learn about environment variables: https://gatsby.dev/env-vars
           Authorization: `Bearer ${process.env.GITHUB_TOKEN}`,
         },
+        // HTTP headers alternatively accepts a function (allows async)
+        headers: async () => {
+          return {
+            Authorization: await getAuthorizationToken(),
+          }
+        },
         // Additional options to pass to node-fetch
         fetchOptions: {},
       },

packages/gatsby-source-graphql/src/gatsby-node.js

@@ -51,8 +51,8 @@ exports.sourceNodes = async (
     link = createHttpLink({
       uri: url,
       fetch,
-      headers,
       fetchOptions,
+      headers: typeof headers === `function` ? await headers() : headers,
     })
   }