feat(aws): Implement the account API for AWS accounts

[dzhengg]

The commit with the fix solves the following issue:
Test amazon account:

> cat test-aws-iam-account.json
{
  "name": "test-aws-iam-account",
  "type": "aws",
  "accountId": "123456789012",
  "regions": [
    {
      "name": "us-west-2"
    },
    {
      "name": "us-east-2"
    }
  ],
  "lambdaEnabled": true,
  "assumeRole": "role/myrole"
}

Before the change:

> curl -X PUT -H "Content-Type: application/json" -H "X-SPINNAKER-USER: account-manager" [email protected] http://localhost:8080/credentials
{"error":"Internal Server Error","exception":"java.lang.NullPointerException","message":"null","status":500,"timestamp":1710223163791}

After the change:

> curl -X PUT -H "Content-Type: application/json" -H "X-SPINNAKER-USER: account-manager" [email protected] http://localhost:8080/credentials
{"type":"aws","accountId":"123456789012","allowPrivateThirdPartyImages":false,"assumeRole":"role/myrole","dedicatedTenancy":false,"enableFips":false,"lambdaEnabled":true,"name":"test-aws-iam-account","permissions":{},"regions":[{"name":"us-east-2"},{"name":"us-west-2"}],"requiredGroupMembership":[]}