Add option to be able to use BCFIPS when BC is blocked in a FIPS environment

[olamy]

When using using registar created for BCFIPS (which means any BC classes are available within the class path).

I got

java.lang.IllegalArgumentException: BouncyCastle not registered
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.ValidateUtils.createFormattedException(ValidateUtils.java:213)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.ValidateUtils.throwIllegalArgumentException(ValidateUtils.java:179)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.ValidateUtils.checkTrue(ValidateUtils.java:156)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.security.SecurityUtils.getBouncycastleEncryptedPrivateKeyInfoDecryptor(SecurityUtils.java:553)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.config.keys.loader.pem.PKCS8PEMResourceKeyPairParser.decryptKeyPairs(PKCS8PEMResourceKeyPairParser.java:107)

I tried to "trick" it :) using the name BC here https://github.com/jenkinsci/mina-sshd-api-plugin/pull/114/files#diff-5440105bdcdf53b86acce84166b9884f497eb6908da1d68b82ec974aa0fd83e1R45

But turns into:

  Caused: java.lang.NoClassDefFoundError: org/bouncycastle/crypto/prng/RandomGenerator
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.security.bouncycastle.BouncyCastleRandomFactory.create(BouncyCastleRandomFactory.java:43)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.util.security.bouncycastle.BouncyCastleRandomFactory.create(BouncyCastleRandomFactory.java:28)
  at PluginClassLoader for mina-sshd-api-common//org.apache.sshd.common.random.SingletonRandomFactory.<init>(SingletonRandomFactory.java:38)

with BCFIPS (classic BC class not available in the classpth) this is this class https://javadoc.io/doc/org.bouncycastle/bc-fips/latest/org/bouncycastle/crypto/fips/FipsSecureRandom.html

But the idea is to make this random factory more configurable.