Add verifymessage feature

[tcharding]

• Patch 1: Adds new feature "verifymessage" fixing by broken master branch.
• Patch 2: Fixes pinning allowing this PR to get past CI.

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In #326 we changed a function to use bitcoin::sign_message::MessageSignature but doing so requires the "base64" feature to be on for bitcoin. This did not get caught by CI but improvements to CI in #338 will now catch this.

Add a feature to json and client that allows enabling "base64" if the verifymessage RPC call is required.

[apoelstra]

Can you switch the commits? Currently the first one doesn't compile.

[tcharding]

Can you switch the commits? Currently the first one doesn't compile.

Oh you build each commit before merging, nice. I have not been doing that when merging things. Lesson learned.

I added feature gating to the import as well which got lost somehow. I thought about it this morning, glad I got to add it.

[apoelstra]

So, I'm unsure about this verifymessage feature. At least, it shouldn't be called verifymessage.

PSBTs are also base64-encoded and this is handled in the PSBT API by just taking PSBTs as &strs. I think we should either do that here, or change the PSBT API to also be feature-gated on base64, or just change the bitcoin dep to always have the base64 feature on.

[tcharding]

or just change the bitcoin dep to always have the base64 feature on.

I'll do that, thanks.

[tcharding]

Done as a separate PR because it is totally different to this so using this PR would make the discussion confusing. That is a new process idea I just had, please comment if its worse.

Thanks for the review too @apoelstra, I know you are travelling and this is not the most exciting thing to be fixing while doing so. Appreciate the input.

[tcharding]

Close this if/when #349 goes in.