feat(signer): Notation-OCI-SHA384-ECDSA platform (#28612)

Adds support for the `Notation-OCI-SHA384-ECDSA` signing profile platform.
Also, refactors the `Platform` class to an enum-like class to allow custom platforms (and prevent blocking users if added platforms are missing on the `enum` declaration).

Closes #28580.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk-testing/framework-integ/test/aws-signer/integ.signing-profile.js.snapshot/aws-cdk-signer-signing-profile.template.json

@@ -0,0 +1,58 @@
+{
+ "Resources": {
+  "SigningProfileLambda4B150CCB": {
+   "Type": "AWS::Signer::SigningProfile",
+   "Properties": {
+    "PlatformId": "AWSLambda-SHA384-ECDSA",
+    "SignatureValidityPeriod": {
+     "Type": "MONTHS",
+     "Value": 135
+    }
+   }
+  },
+  "SigningProfileOCI1EA741C3": {
+   "Type": "AWS::Signer::SigningProfile",
+   "Properties": {
+    "PlatformId": "Notation-OCI-SHA384-ECDSA",
+    "SignatureValidityPeriod": {
+     "Type": "DAYS",
+     "Value": 60
+    }
+   }
+  }
+ },
+ "Parameters": {
+  "BootstrapVersion": {
+   "Type": "AWS::SSM::Parameter::Value<String>",
+   "Default": "/cdk-bootstrap/hnb659fds/version",
+   "Description": "Version of the CDK Bootstrap resources in this environment, automatically retrieved from SSM Parameter Store. [cdk:skip]"
+  }
+ },
+ "Rules": {
+  "CheckBootstrapVersion": {
+   "Assertions": [
+    {
+     "Assert": {
+      "Fn::Not": [
+       {
+        "Fn::Contains": [
+         [
+          "1",
+          "2",
+          "3",
+          "4",
+          "5"
+         ],
+         {
+          "Ref": "BootstrapVersion"
+         }
+        ]
+       }
+      ]
+     },
+     "AssertDescription": "CDK bootstrap stack version 6 required. Please run 'cdk bootstrap' with a recent version of the CDK CLI."
+    }
+   ]
+  }
+ }
+}