Change EdgeData string fields to enumerations

[scordio]

The proto and connState string fields of the EdgeData case class should have a well-defined set of values, so their type could be changed to Enumeration.

[scordio]

These could be useful to gather the set of values:

• https://www.bro.org/sphinx/scripts/base/protocols/conn/main.bro.html#type-Conn::Info
• https://www.bro.org/sphinx/scripts/base/init-bare.bro.html#type-transport_proto