Dry run with mock sigs (backport #4832)

[mergify]

Describe your changes

Closes #4714

• When dry-running, dummy signatures are used in txs
• Validation does not perform signature checks when dry-running

I have tested that the gas estimation hasn't changed between dry-running with signatures and with dummies. It is curious however that in both cases there is a small discrepancy between dry-running and the actual tx.

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:

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This is an automatic backport of pull request #4832 done by [Mergify](https://mergify.com).

[mergify]

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