feat(shared): support multiple credential locations (#3407)

### Motivation

We have a number of configuration files to allow reading and writing of
S3 buckets across accounts, sometimes it's useful to join a collection
of access lists together to provide a more comprehensive list of
credentials for the calling service.

### Modifications

Allow credentials to be loaded from multiple sources using either a
comma separated list or a JSON list, this behavior is the same as the
workflows inside of argo

https://github.com/linz/argo-tasks/blob/master/src/fs.register.ts#L132

### Verification

Logic is taken from argo tasks, and will be tested in argo.

packages/shared/src/file.system.ts

@@ -54,8 +54,24 @@ credentials.onFileSystemFound = (acc: AwsCredentialConfig, fs?: FsAwsS3, path?:
   fsa.register(acc.prefix, fs);
 };
 
+/**
+ * Split JSON or comma separated lists into individual components
+ *
+ * Allowing for credentials to be loaded from multiple sources eg
+ *
+ * @example comma separated list
+ * ```typescript
+ * "s3://foo/bar.json,s3://foo/baz.json"
+ * ```
+ */
+function splitConfig(x: string): string[] {
+  if (x.startsWith('[')) return JSON.parse(x) as string[];
+  return x.split(',');
+}
 const credentialPath = Env.get(Env.AwsRoleConfigPath);
-if (credentialPath) credentials.registerConfig(fsa.toUrl(credentialPath), s3Fs);
+if (credentialPath) {
+  for (const loc of splitConfig(credentialPath)) credentials.registerConfig(fsa.toUrl(loc), s3Fs);
+}
 
 s3Fs.credentials = credentials;