feat: add borsh utilities and public key support (#17239)

* feat: add borsh utilities and public key support

* fix: make bn internal for flow

* fix: add Buffer import in borsh file

web3.js/src/util/borsh-schema.ts

@@ -0,0 +1,34 @@
+import {Buffer} from 'buffer';
+import {serialize, deserialize} from 'borsh';
+
+// Class wrapping a plain object
+export class Struct {
+  constructor(properties: any) {
+    Object.assign(this, properties);
+  }
+
+  encode(): Buffer {
+    return Buffer.from(serialize(SOLANA_SCHEMA, this));
+  }
+
+  static decode(data: Buffer): any {
+    return deserialize(SOLANA_SCHEMA, this, data);
+  }
+}
+
+// Class representing a Rust-compatible enum, since enums are only strings or
+// numbers in pure JS
+export class Enum extends Struct {
+  enum: string = '';
+  constructor(properties: any) {
+    super(properties);
+    if (Object.keys(properties).length !== 1) {
+      throw new Error('Enum can only take single value');
+    }
+    Object.keys(properties).map(key => {
+      this.enum = key;
+    });
+  }
+}
+
+export const SOLANA_SCHEMA: Map<Function, any> = new Map();