chore: add tx type constants

[mattsse]

add constants, will be required by #1498 for example

cc @chirag-bgh

[codecov-commenter]

Codecov Report

Merging #1639 (fbc6109) into main (31f62c1) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main    #1639      +/-   ##
==========================================
- Coverage   74.44%   74.42%   -0.02%     
==========================================
  Files         377      377              
  Lines       45197    45197              
==========================================
- Hits        33647    33639       -8     
- Misses      11550    11558       +8     

Flag	Coverage Δ
integration-tests	20.95% <0.00%> (+<0.01%)	⬆️
unit-tests	69.02% <0.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
crates/primitives/src/lib.rs	100.00% <ø> (ø)
crates/primitives/src/transaction/mod.rs	90.02% <ø> (ø)
crates/primitives/src/transaction/tx_type.rs	70.83% <0.00%> (ø)
crates/net/discv4/src/proto.rs	89.66% <0.00%> (-3.13%)	⬇️
crates/net/downloaders/src/bodies/task.rs	95.45% <0.00%> (-1.14%)	⬇️
crates/net/network/src/session/active.rs	84.59% <0.00%> (-0.75%)	⬇️
crates/net/eth-wire/src/capability.rs	74.26% <0.00%> (-0.74%)	⬇️
crates/net/discv4/src/lib.rs	65.95% <0.00%> (-0.15%)	⬇️
crates/net/downloaders/src/headers/task.rs	83.67% <0.00%> (ø)
crates/net/network/src/session/mod.rs	77.94% <0.00%> (+1.35%)	⬆️
... and 1 more

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