Add changelog

.changelog/unreleased/features/2260-wallet-cli-revamping-main-rebased.md

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+- The wallet CLI structure has been significantly reworked and simplified.
+  Alias argument is now obligatory for key generation / derivation
+  commands. Feature of raw (non-HD) key generation has been restored,
+  which was removed in the previous release. Key export / import
+  functionality for both transparent and shielded key kinds has been
+  implemented. Additionally, several other improvements have been made.
+  ([\#2260](https://github.com/anoma/namada/pull/2260))