feat(ctb): make Proxy inheritable

Makes the Proxy contract inheritable by making external functions into
(public virtual) functions. Does not impact generated bytecode or gas
snapshot.

.changeset/olive-students-love.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Makes the Proxy contract inheritable by making functions (public virtual).

packages/contracts-bedrock/contracts/universal/Proxy.sol

@@ -84,7 +84,7 @@ contract Proxy {
      *
      * @param _implementation Address of the implementation contract.
      */
-    function upgradeTo(address _implementation) external proxyCallIfNotAdmin {
+    function upgradeTo(address _implementation) public virtual proxyCallIfNotAdmin {
         _setImplementation(_implementation);
     }
 
@@ -96,8 +96,9 @@ contract Proxy {
      * @param _data           Calldata to delegatecall the new implementation with.
      */
     function upgradeToAndCall(address _implementation, bytes calldata _data)
-        external
+        public
         payable
+        virtual
         proxyCallIfNotAdmin
         returns (bytes memory)
     {
@@ -112,7 +113,7 @@ contract Proxy {
      *
      * @param _admin New owner of the proxy contract.
      */
-    function changeAdmin(address _admin) external proxyCallIfNotAdmin {
+    function changeAdmin(address _admin) public virtual proxyCallIfNotAdmin {
         _changeAdmin(_admin);
     }
 
@@ -121,7 +122,7 @@ contract Proxy {
      *
      * @return Owner address.
      */
-    function admin() external proxyCallIfNotAdmin returns (address) {
+    function admin() public virtual proxyCallIfNotAdmin returns (address) {
         return _getAdmin();
     }
 
@@ -130,7 +131,7 @@ contract Proxy {
      *
      * @return Implementation address.
      */
-    function implementation() external proxyCallIfNotAdmin returns (address) {
+    function implementation() public virtual proxyCallIfNotAdmin returns (address) {
         return _getImplementation();
     }