Override sim2h_url with DNA property

[lucksus]

PR summary

This adds a special DNA property sim2h_url that, if set, overrides the conductor wide setting for the network configuration variable sim2h_url.

This enables using different sim2h servers for different DNAs, all running in the same conductor.

Example of setting the DNA property:

{
  "name": "My Example DNA",
  "description": "This DNA shows how to override the conductor wide sim2h URL by DNA",
  "version": "0.1",
  "uuid": "00000000-0000-0000-0000-000000000000",
  "dna_spec_version": "2.0",
  "properties": {
      "sim2h_url": "wss://sim2h.my-example-server.cominvald:9000"
  }
}

Fixed handling of properties

hc package was not including DNA properties mentioned in JSON manifest files as shown in the example above. Instead it was overwriting properties with extra properties that could be added during packaging step.

This PR includes a fix that merges both sources.

changelog

• if this is a code change that effects some consumer (e.g. zome developers) of holochain core, then it has been added to our between-release changelog with the format

- summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

documentation

• this code has been documented according to our docs checklist

[zippy]

woot