[Icon] Add Metafields icon (#7012)

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### WHY are these changes introduced?

Add new Metafields icon to Polaris that is currently up in Admin Settings.

### WHAT is this pull request doing?

<img width="477" alt="Screen Shot 2022-08-24 at 2 28 38 PM" src="https://user-images.githubusercontent.com/99364596/186517098-f74c3746-bad8-4ace-8b49-be75d800d41c.png">

Admin settings is currently using the latest Metafields icon, but it is not available in Polaris.

Created a Major and Minor of the icon.

<img width="401" alt="Screen Shot 2022-08-24 at 2 31 06 PM" src="https://user-images.githubusercontent.com/99364596/186517484-157fbae3-5fe9-4a03-b177-764f5f52db94.png">

Co-authored-by: Alex Page <hi@alexpage.dev>

Author: leileu

.changeset/nervous-mayflies-juggle.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris-icons': major
+---
+
+Adding Metafields icon to polaris

polaris-icons/icons/MetafieldsMajor.svg

@@ -0,0 +1,12 @@
+name: metafields
+set: major
+description: Used to represent metafields
+keywords:
+  - metafields
+  - data sources
+authors:
+  - Leina Leung
+version: 1
+date_added: 2022-08-24
+date_modified: 2022-08-24
+exclusive_use:

polaris-icons/icons/MetafieldsMajor.yml

@@ -0,0 +1,12 @@
+name: metafields
+set: minor
+description: Used to represent metafields
+keywords:
+  - metafields
+  - data sources
+authors:
+  - Leina Leung
+version: 1
+date_added: 2022-08-24
+date_modified: 2022-08-24
+exclusive_use: