Refactor pgp::mpi to use std::vector instead of fixed-size buffer.

[ni4]

Less memory usage, more robust, cleaner code.

[codecov]

Codecov Report

Attention: Patch coverage is 96.63300% with 10 lines in your changes missing coverage. Please review.

Project coverage is 85.33%. Comparing base (ccbd02e) to head (1033e23).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib/crypto/mpi.cpp	92.30%	2 Missing ⚠️
src/lib/crypto/ossl_utils.hpp	60.00%	2 Missing ⚠️
src/lib/crypto/rsa.cpp	87.50%	2 Missing ⚠️
src/lib/crypto/botan_utils.hpp	83.33%	1 Missing ⚠️
src/lib/crypto/ecdsa.cpp	90.90%	1 Missing ⚠️
src/lib/crypto/rsa_ossl.cpp	93.75%	1 Missing ⚠️
src/lib/crypto/sm2.cpp	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2338      +/-   ##
==========================================
+ Coverage   85.27%   85.33%   +0.06%     
==========================================
  Files         125      125              
  Lines       22814    22841      +27     
==========================================
+ Hits        19454    19491      +37     
+ Misses       3360     3350      -10     

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[maxirmx]

LGTM

[maxirmx]

Merging with two approvals