Add Feature Request, Good First Issue, and General Issue Templates (#891)

Followup to #758. Adds the following issue templates mostly from their
Bitcoin Core counterparts (excepting the "General" template):

- [x] [Feature
Request](https://github.com/bitcoin/bitcoin/blob/master/.github/ISSUE_TEMPLATE/feature_request.yml)
- [x] [Good First
Issue](https://github.com/bitcoin/bitcoin/blob/master/.github/ISSUE_TEMPLATE/good_first_issue.yml)
- [x] General

Author: node-smithxby72w

.github/ISSUE_TEMPLATE/feature-request.yml

@@ -0,0 +1,46 @@
+name: Feature Request
+description: Suggest an idea for this project.
+labels: [Feature]
+body:
+  - type: dropdown
+    attributes:
+      label: Which crate is this feature request for?
+      multiple: true
+      options:
+        - payjoin
+        - payjoin-cli
+        - payjoin-directory
+        - payjoin-test-utils
+        - payjoin-ffi
+  - type: textarea
+    id: feature
+    attributes:
+      label: Please describe the feature you'd like to see added.
+      description: Attach screenshots or logs if applicable.
+    validations:
+      required: true
+  - type: textarea
+    id: related-problem
+    attributes:
+      label: Is your feature related to a problem, if so please describe it.
+      description: Attach screenshots or logs if applicable.
+    validations:
+      required: false
+  - type: textarea
+    id: solution
+    attributes:
+      label: Describe the solution you'd like
+    validations:
+      required: false
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Describe any alternatives you've considered
+    validations:
+      required: false
+  - type: textarea
+    id: additional-context
+    attributes:
+      label: Please leave any additional context
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/general.yml

@@ -0,0 +1,20 @@
+name: General Question / Support
+description: A general question or support issue that isn't a bug or feature.
+body:
+  - type: dropdown
+    attributes:
+      label: Which crate is this feature request for (if any)?
+      multiple: true
+      options:
+        - payjoin
+        - payjoin-cli
+        - payjoin-directory
+        - payjoin-test-utils
+        - payjoin-ffi
+  - type: textarea
+    id: question
+    attributes:
+      label: Question
+      description: Describe the issue
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/good-first-issue.yml

@@ -0,0 +1,49 @@
+name: Good First Issue
+description: (Regular devs only) Suggest a new good first issue
+labels: [good first issue]
+body:
+  - type: dropdown
+    attributes:
+      label: Which crate is this issue for?
+      multiple: true
+      options:
+        - payjoin
+        - payjoin-cli
+        - payjoin-directory
+        - payjoin-test-utils
+        - payjoin-ffi
+  - type: markdown
+    attributes:
+      value: |
+        Please add the label "good first issue" manually before or after opening
+
+        A good first issue is an uncontroversial issue, that has a relatively unique and obvious solution
+
+        Motivate the issue and explain the solution briefly
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Motivation
+      description: Motivate the issue
+    validations:
+      required: true
+  - type: textarea
+    id: solution
+    attributes:
+      label: Possible solution
+      description: Describe a possible solution
+    validations:
+      required: false
+  - type: textarea
+    id: useful-skills
+    attributes:
+      label: Useful Skills
+      description: For example, “Familiarity with BIP 77”.
+  - type: textarea
+    attributes:
+      label: Guidance for new contributors
+      description: Please leave this to automatically add the footer for new contributors
+      value: |
+        Want to work on this issue?
+
+        For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/payjoin/rust-payjoin/blob/master/.github/CONTRIBUTING.md) before opening your pull request.