a032fa30d2 multiprocess: add interfaces::ExternalSigner class (Russell Yanofsky)
Pull request description:
Add `interfaces::ExternalSigner` class to let signer objects be passed between processes and let signer code run in the original process where the object was created.
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This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10).
ACKs for top commit:
laanwj:
Concept and code review ACK a032fa30d2
hebasto:
re-ACK a032fa30d2
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