Merge bitcoindevkit/bdk#477: Update issue templates

8534cd39433211733c325a73ca15fe7d569e8401 Update issue templates (Steve Myers)

Pull request description:

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  ### Description

  Add bug reporting issue template and template for proposing a "Summer of Bitcoin" project.

  ### Notes to the reviewers

  The SoB template is basically a copy of what Adi created but lightly formatted to work as github issue templates.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

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