feat: support pull requests

Support for pull requests, it's not standard in NIPs yet[1] but there must be
two clients that support it to be included in NIPs, n34 is one of them.

This support:
- Create a new PR `n34 pr new`
- Update PR tip `n34 pr update`
- View a PR `n34 pr view`
- List PRs `n34 pr list`
- Add `PR` and `PR-update` kinds as roots in `NostrClient::find_root`
- Support issuing statuses for PRs
- Support GRASP PR and PR-update
- Update the docs

[1] https://github.com/nostr-protocol/nips/pull/1966

docs/pr/merge.md

@@ -0,0 +1,22 @@
+# Merge a Pull Request
+
+> `n34 pr merge` command
+
+**Usage:**
+```
+Usage: n34 pr merge [OPTIONS] <PR_ID> <MERGE_COMMIT>
+
+Arguments:
+  <PR_ID>         The open PR id to merge it
+  <MERGE_COMMIT>  The merge commit id
+
+Options:
+      --repo <NADDR-NIP05-OR-SET>  Repository addresses
+```
+
+Creates a kind `1631` event (Applied/Merged status) for the specified PR. The
+PR must be in open status.
+
+You can get the `MERGE_COMMIT` commit using `git rev-parse HEAD` command if
+the merge commit in the `HEAD` or use `HEAD~n` where the `n` is the number of
+commits the merge commit before the HEAD