builtins: Add AWS Sig v4 signing builtin. (#5376)

This commit adds initial support for AWS's SigV4 request signing system,
which will allow OPA's existing `http.send` builtin to be used to more
conveniently query cloud resources. It automates away most of the pain
around signing the request headers and body, and is designed to compose
with `http.send` directly.

Internally, this also refactors AWS SigV4 request signing, so that the
signing logic is shared between the builtin and the REST plugin for AWS.

Fixes: #3749

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>

capabilities.json

@@ -3074,6 +3074,50 @@
         "type": "function"
       }
     },
+    {
+      "name": "providers.aws.sign_req",
+      "decl": {
+        "args": [
+          {
+            "dynamic": {
+              "key": {
+                "type": "string"
+              },
+              "value": {
+                "type": "any"
+              }
+            },
+            "type": "object"
+          },
+          {
+            "dynamic": {
+              "key": {
+                "type": "string"
+              },
+              "value": {
+                "type": "any"
+              }
+            },
+            "type": "object"
+          },
+          {
+            "type": "number"
+          }
+        ],
+        "result": {
+          "dynamic": {
+            "key": {
+              "type": "any"
+            },
+            "value": {
+              "type": "any"
+            }
+          },
+          "type": "object"
+        },
+        "type": "function"
+      }
+    },
     {
       "name": "rand.intn",
       "decl": {