[APM] consider Transaction.marks.agent an option property

[hmdhk]

Transaction.marks.agent is an optional property and might be missing in certain types of Transactions.

The APM UI currently considers this property as required and throws an TypeError: "can't convert undefined to object" if it's missing.

[elasticmachine]

Pinging @elastic/apm-ui

[sorenlouv]

agent should be marked as optional in:

https://github.com/sqren/kibana/blob/f0dad8bc1451e3e32487bb1473d472e7a57491ed/x-pack/plugins/apm/typings/Transaction.ts#L39-L43

like:

interface Marks {
  agent?: {
    [name: string]: number;
  };
}