SDK+client/PoS: allow to set a receiver target for claim reward tx

[tzemanovic]

Describe your changes

Follow-up to #4402 (based on it - diff https://github.com/anoma/namada/pull/4427/files/9b01f9a79e7c34a17102d7bafc7c040ce3ccab83..54d1162d28f5e7f007e8a5f4386f8f6e3259e635).

This allow to optionally set the rewards target in the SDK and the client and it should only be used once the updated tx is deployed on-chain.

Checklist before merging

• [ ] If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• [ ] If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• [ ] If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies: