feat: support read messages from outbox level

[zcabter]

Closes RV-896

What

Support reading outbox messages from an outbox level.

Why

The Outbox component needs to expose this API to support get_outbox API in the Rollup Node.

Manually Testing

make all

Regressions

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Explain changes to regression test captures when applicable.
• Write commit messages in agreement with our guidelines.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.

[github-actions]

Benchmark results for revision 57deeb6:

Metric	Duration	TPS
Mean	1.526054748s	26.212
Worst	1.538588451s	25.998
Best	1.519380682s	26.327
Standard Deviation	±4.730553ms	±0.081

Full results

Run	Transfers	Duration	TPS
1	40	1.525976339s	26.213
2	40	1.524567323s	26.237
3	40	1.530490225s	26.135
4	40	1.528349766s	26.172
5	40	1.525802819s	26.216
6	40	1.52143352s	26.291
7	40	1.523292646s	26.259
8	40	1.521051858s	26.298
9	40	1.520162663s	26.313
10	40	1.528607328s	26.168
11	40	1.531468448s	26.119
12	40	1.524295461s	26.242
13	40	1.523363279s	26.258
14	40	1.519380682s	26.327
15	40	1.522532097s	26.272
16	40	1.523379951s	26.257
17	40	1.538588451s	25.998
18	40	1.525125166s	26.227
19	40	1.530723725s	26.131
20	40	1.532503213s	26.101

Compare the results above with those for the default branch.

[codecov]

Codecov Report

❌ Patch coverage is 94.50549% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.09%. Comparing base (440d180) to head (469a506).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/riscv/lib/src/pvm/outbox.rs	94.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #793      +/-   ##
==========================================
+ Coverage   91.07%   91.09%   +0.02%     
==========================================
  Files         110      110              
  Lines       20831    20913      +82     
  Branches    20831    20913      +82     
==========================================
+ Hits        18971    19050      +79     
- Misses       1487     1490       +3     
  Partials      373      373              

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