[IMPLEMENTATION] RSA algorithm

[ddaniel27]

This is my implementation for RSA crypto algorithm. There is another implementation in the repo, but that file just contains the encryption/decryption algorithm assuming that we have the private/public key and modulus. My implementation contains:

• RSA struct to easily manage keys and methods.
• Keys Generation (with random prime numbers selection).
• String encryption/decryption.
• Number encryption/decryption.
• Getters for both (public key, modulus) and (private key).

Tests and benchmarks are included :)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 87.49%. Comparing base (cddedb6) to head (6db6526).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
cipher/rsa/rsa2.go	84.00%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #727      +/-   ##
==========================================
- Coverage   87.53%   87.49%   -0.04%     
==========================================
  Files         204      205       +1     
  Lines        5389     5439      +50     
==========================================
+ Hits         4717     4759      +42     
- Misses        533      539       +6     
- Partials      139      141       +2     

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