Fix sim2h dependence on agent signing for hosted hApps use-case

[zippy]

PR summary

In the use-case of a remote signing service that is not always guaranteed to be available (as in the hosted happs usecase of holo), assumptions in sim2h for message integrity that rely on agents signing all messages, breaks down.

This PR fixes this issue by only requiring that the Join message be signed by the agent. This works along with wss because the connection (which is long lasting) is then secured by the tls layer.

• convert panic to error on sim2h_worker signing fail
• sign only Join messages
• add explicit SigningServiceError for dna's to handle
• add back wss to ensure connection integrity

testing/benchmarking notes

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followups

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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