fix(rpc): make exp claim optional

[Rjected]

The exp claim is not required, and is not sent in engine API hive tests. So it is made optional.

Fixes #1587

[codecov-commenter]

Codecov Report

Merging #1601 (efe9c5b) into main (2884eae) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1601      +/-   ##
==========================================
- Coverage   74.81%   74.80%   -0.01%     
==========================================
  Files         374      374              
  Lines       44249    44411     +162     
==========================================
+ Hits        33105    33222     +117     
- Misses      11144    11189      +45     

Flag	Coverage Δ
integration-tests	21.03% <0.00%> (-0.10%)	⬇️
unit-tests	69.46% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
crates/rpc/rpc/src/layers/auth_layer.rs	99.20% <100.00%> (ø)
crates/rpc/rpc/src/layers/jwt_secret.rs	96.86% <100.00%> (+0.17%)	⬆️
crates/net/network/src/session/mod.rs	76.59% <0.00%> (-1.36%)	⬇️
crates/net/downloaders/src/headers/task.rs	83.67% <0.00%> (-1.03%)	⬇️
crates/stages/src/stages/sender_recovery.rs	90.95% <0.00%> (-0.54%)	⬇️
crates/net/eth-wire/src/ethstream.rs	83.97% <0.00%> (-0.48%)	⬇️
crates/stages/src/stages/hashing_storage.rs	95.62% <0.00%> (-0.37%)	⬇️
crates/stages/src/stages/merkle.rs	77.70% <0.00%> (ø)
crates/primitives/src/chain/spec.rs	97.87% <0.00%> (ø)
crates/executor/src/execution_result.rs
... and 11 more

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