Move encode/decode type key functions to openscreen-common

[backkem]

Context

From PR #13 review comment by @kaidokert:

These end up duplicated now. I'd probably move them over to openscreen-common crate and just import from there.

Problem

The encode_type_key and decode_type_key functions are duplicated across:

• openscreen-application/src/messages.rs
• openscreen-network/src/messages.rs

Proposed Solution

Move these functions to the openscreen-common crate and import them from both openscreen-application and openscreen-network.

[backkem]

This may be one that Jules can handle 🤔