Change the default returning type of 'to_cbor' to bytes

[cffls]

Change the default returning type of 'to_cbor' to bytes and add 'to_cbor_hex' that returns cbor hex.

[codecov-commenter]

Codecov Report

Merging #232 (2da68c0) into main (40f00fb) will increase coverage by 0.04%.
The diff coverage is 90.47%.

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@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
+ Coverage   85.45%   85.50%   +0.04%     
==========================================
  Files          26       26              
  Lines        2957     2953       -4     
  Branches      709      707       -2     
==========================================
- Hits         2527     2525       -2     
+ Misses        318      317       -1     
+ Partials      112      111       -1     

Impacted Files	Coverage Δ
pycardano/key.py	87.83% <ø> (ø)
pycardano/metadata.py	94.59% <ø> (ø)
pycardano/utils.py	94.82% <ø> (ø)
pycardano/backend/base.py	93.58% <66.66%> (ø)
pycardano/txbuilder.py	90.33% <75.00%> (ø)
pycardano/nativescript.py	98.01% <100.00%> (ø)
pycardano/plutus.py	87.60% <100.00%> (ø)
pycardano/serialization.py	85.08% <100.00%> (+0.46%)	⬆️
pycardano/transaction.py	94.09% <100.00%> (ø)