feat: expand GetLastPendingCertificates

[atanmarko]

Description

Expand TCE grpc api function GetLastPendingCertificates result with index of the last pending certificate.

Fixes TP-695

Breaking changes

This is a breaking change for GRPC api, it needs to be rebuilt.

PR Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added or updated tests that comprehensively prove my change is effective or that my feature works

[codecov]

Codecov Report

Patch coverage: 5.66% and project coverage change: -0.11% ⚠️

Comparison is base (06a8e1c) 53.40% compared to head (cb63a66) 53.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   53.40%   53.30%   -0.11%     
==========================================
  Files         214      214              
  Lines       10330    10360      +30     
==========================================
+ Hits         5517     5522       +5     
- Misses       4813     4838      +25     

Files Changed	Coverage Δ
crates/topos-sequencer-subnet-runtime/src/lib.rs	0.00% <0.00%> (ø)
crates/topos-sequencer-subnet-runtime/src/proxy.rs	0.00% <0.00%> (ø)
crates/topos-sequencer/src/app_context.rs	0.00% <0.00%> (ø)
crates/topos-sequencer/src/lib.rs	0.00% <0.00%> (ø)
crates/topos-tce-api/src/grpc/mod.rs	55.11% <0.00%> (-2.30%)	⬇️
crates/topos-tce-api/src/runtime/commands.rs	0.00% <ø> (ø)
crates/topos-tce-proxy/src/client.rs	0.00% <0.00%> (ø)
crates/topos-tce-proxy/src/worker.rs	0.00% <0.00%> (ø)
crates/topos-tce-proxy/tests/tce_tests.rs	42.46% <ø> (ø)
crates/topos-tce/src/app_context/api.rs	57.57% <0.00%> (-2.43%)	⬇️
... and 2 more

... and 2 files with indirect coverage changes

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