request for a new transport field in source / client / server

Folks,

I'm wondering if you could consider including a new field, `transport`, in the source / client / serve specifications.

There are several application protocols, for example SIP or DOTS, that use multiple transport protocols (UDP, TCP, SCTP). The field would allow to discriminate flows over different transport protocols.

The actual specification could be the same as in `network`.

```
- name: transport
level: core
type: keyword
short: Protocol Name corresponding to the field `iana_number`.
description: >
Same as network.iana_number, but instead using the Keyword name of the
transport layer (udp, tcp, ipv6-icmp, etc.)

The field value must be normalized to lowercase for querying. See
the documentation section "Implementing ECS".
example: tcp
```