Extract route paths prefixes into constants

[willyborankin]

Extracted route path prefixes into contants:

• /_plugins/_security - PLUGIN_ROUTE_PREFIX
• /_opendistro/_security - LEGACY_PLUGIN_ROUTE_PREFIX
• /_plugins/_security/api - PLUGIN_API_ROUTE_PREFIX
• /_opendistro/_security/api - LEGACY_PLUGIN_API_ROUTE_PREFIX

Same part of #4153 for simplicity of the final PR.

Check List

• New functionality includes testing
• New functionality has been documented
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 66.23%. Comparing base (b0d26dd) to head (e73d1fc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4173      +/-   ##
==========================================
+ Coverage   66.22%   66.23%   +0.01%     
==========================================
  Files         302      302              
  Lines       21698    21701       +3     
  Branches     3501     3501              
==========================================
+ Hits        14369    14374       +5     
+ Misses       5577     5575       -2     
  Partials     1752     1752              

Files	Coverage Δ
...y/action/onbehalf/CreateOnBehalfOfTokenAction.java	12.90% <ø> (ø)
...g/opensearch/security/dlic/rest/support/Utils.java	63.41% <100.00%> (ø)
...opensearch/security/rest/DashboardsInfoAction.java	79.54% <100.00%> (+0.47%)	⬆️
...opensearch/security/rest/SecurityHealthAction.java	100.00% <ø> (ø)
...g/opensearch/security/rest/SecurityInfoAction.java	51.85% <ø> (ø)
...org/opensearch/security/rest/TenantInfoAction.java	86.95% <ø> (ø)
...dlic/auth/http/saml/AuthTokenProcessorHandler.java	50.28% <50.00%> (-0.01%)	⬇️

... and 4 files with indirect coverage changes

[stephen-crawford]

nice