[Backport 2.x] Use boucycastle PEM reader instead of reg expression (#2864)

[willyborankin]

Backpor 3c9f7c2 from #2864

[codecov]

Codecov Report

Merging #2877 (7f00a73) into 2.x (0d6ac30) will decrease coverage by 0.10%.
The diff coverage is 54.54%.

@@             Coverage Diff              @@
##                2.x    #2877      +/-   ##
============================================
- Coverage     61.53%   61.43%   -0.10%     
+ Complexity     3386     3377       -9     
============================================
  Files           264      264              
  Lines         18665    18645      -20     
  Branches       3288     3287       -1     
============================================
- Hits          11485    11455      -30     
- Misses         5597     5603       +6     
- Partials       1583     1587       +4     

Impacted Files	Coverage Δ
.../org/opensearch/security/support/PemKeyReader.java	74.59% <54.54%> (-2.18%)	⬇️

... and 4 files with indirect coverage changes

[cwperks]

@willyborankin Can you fix the unused import error?

Error: eckstyle] [ERROR] /home/runner/work/security/security/src/main/java/org/opensearch/security/support/PemKeyReader.java:30:8: Unused import - java.io.ByteArrayOutputStream. [UnusedImports]
Error: eckstyle] [ERROR] /home/runner/work/security/security/src/main/java/org/opensearch/security/support/PemKeyReader.java:36:8: Unused import - java.io.OutputStream. [UnusedImports]

[willyborankin]

@willyborankin Can you fix the unused import error?

Error: eckstyle] [ERROR] /home/runner/work/security/security/src/main/java/org/opensearch/security/support/PemKeyReader.java:30:8: Unused import - java.io.ByteArrayOutputStream. [UnusedImports]
Error: eckstyle] [ERROR] /home/runner/work/security/security/src/main/java/org/opensearch/security/support/PemKeyReader.java:36:8: Unused import - java.io.OutputStream. [UnusedImports]

done