Offline signatures from SDK

[grarco]

Describe your changes

This PR moves the offline signing logic from the client to the SDK.
Also removes some remaining traces of SignTx which is a duplicated of OfflineSign (I had already started removing it in #4120 but forgot to remove everything).

I've also opened #4235 with some followup tasks on this topic.

cc: @jurevans

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:

[codecov]

Codecov Report

Attention: Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Project coverage is 74.59%. Comparing base (c0b9f68) to head (bc68286).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/sdk/src/signing.rs	86.84%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4234   +/-   ##
=======================================
  Coverage   74.59%   74.59%           
=======================================
  Files         342      342           
  Lines      108724   108762   +38     
=======================================
+ Hits        81101    81132   +31     
- Misses      27623    27630    +7     

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[brentstone]

@tzemanovic @grarco adding this to the v1.1.0 milestone - any concerns here?