ACM-PCA: Use serialisable types instead of native objs

[viren-nadkarni]

This is a follow up to https://github.com/getmoto/moto/pull/7958

The ImportCertificateAuthorityCertificate would create a cryptography.hazmat.bindings._rust.x509.Certificate object as assign it as a CertificateAuthority class member. This prevents it from being serialised with pickle.

This PR reworks the certificate and private key attributes so that all members are picklable.

[bblommers]

Hey @viren-nadkarni! Do you want to add some tests for this as well in Moto? Just to prevent it from breaking again in the future

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.43%. Comparing base (191ebcc) to head (ecfa1b3). Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8039   +/-   ##
=======================================
  Coverage   94.43%   94.43%           
=======================================
  Files        1139     1139           
  Lines       97355    97387   +32     
=======================================
+ Hits        91935    91966   +31     
- Misses       5420     5421    +1     

Flag	Coverage Δ
servertests	28.98% <36.36%> (+0.05%)	:arrow_up:
unittests	94.40% <100.00%> (+<0.01%)	:arrow_up:

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