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 gatsbyjs/gatsby#20731. Updated README.md to
include new (optional) syntax.

yarn.lock

@@ -7810,12 +7810,12 @@ lodash.uniq@^4.5.0:
   resolved "https://registry.yarnpkg.com/lodash.uniq/-/lodash.uniq-4.5.0.tgz#d0225373aeb652adc1bc82e4945339a842754773"
   integrity sha1-0CJTc662Uq3BvILklFM5qEJ1R3M=
 
-lodash@^4.11.1, lodash@^4.13.1, lodash@^4.15.0, lodash@^4.17.11, lodash@^4.17.13, lodash@^4.17.14, lodash@^4.17.4, lodash@^4.17.5, lodash@^4.3.0:
+lodash@^4.11.1, lodash@^4.13.1, lodash@^4.15.0, lodash@^4.17.11, lodash@^4.17.13, lodash@^4.17.4, lodash@^4.17.5, lodash@^4.3.0:
   version "4.17.14"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.14.tgz#9ce487ae66c96254fe20b599f21b6816028078ba"
   integrity sha512-mmKYbW3GLuJeX+iGP+Y7Gp1AiGHGbXHCOh/jZmrawMmsE7MS4znI3RL2FsjbqOyMayHInjOeykW7PEajUk1/xw==
 
-lodash@^4.17.15:
+lodash@^4.17.14, lodash@^4.17.15:
   version "4.17.15"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.15.tgz#b447f6670a0455bbfeedd11392eff330ea097548"
   integrity sha512-8xOcRHvCjnocdS5cpwXQXVzmmh5e5+saE2QGoeQmbKmRS6J3VQppPOIt0MnmE+4xlZoumy0GPG0D0MVIQbNA1A==