Generalize public key reading in the JWT authenticator

[cwperks]

Description

This PR generalizes public key reading in the JWT authenticator to support configuring the JWT authenticator with inline public cert like this:

jwt_auth_domain:
  description: "Authenticate via Json Web Token"
  http_enabled: true
  transport_enabled: true
  order: 0
  http_authenticator:
    type: jwt
    challenge: false
    config:
      signing_key: |-
        -----BEGIN PUBLIC KEY-----
        MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA64L6PDUm1ySNWIJiuxEv
        OA92MDmrevshZEWQsN98pkrp8pbCOVjA5OseddXOYVlXunbv1FMooGwVkOCAk6s3
        rxffHP2tN4NJRvvEDr/uoWKj22mV9cqyVar13HvdKkTm0MN1nDjo1fv1YBL5Xq7E
        2Ak6d0soKIg/RFzzv7JqLkUCsQ5krhm08A11cBbI8jqsebsiRCa7HkRyUalYLRVh
        NHf9Dxf3rmVoKtHs1jKRMm9NiAl1aa6U/BDzlM45uX9yNaOYiqNVbQIufwxiy4/a
        gbmkvuqTf/Pm6jzX09h0nb7CIiy+3AbbnncaZ8UjwKZ8iJJ8XTDtHuerM8eN728F
        NwIDAQAB
        -----END PUBLIC KEY-----
      jwt_header: "Authorization"

Currently, the JWT authenticator will fail to initialize because it is not removing the line endings when parsing this value. The resulting error was:

2024-06-05 10:54:13 [2024-06-05T16:54:13,013][ERROR][c.a.d.a.h.j.HTTPJwtAuthenticator] [opensearch-node1] Error while creating JWT authenticator
2024-06-05 10:54:13 java.lang.IllegalArgumentException: Illegal base64 character a
2024-06-05 10:54:13 at java.base/java.util.Base64$Decoder.decode0(Base64.java:852) ~[?:?]
2024-06-05 10:54:13 at java.base/java.util.Base64$Decoder.decode(Base64.java:570) ~[?:?]
2024-06-05 10:54:13 at java.base/java.util.Base64$Decoder.decode(Base64.java:593) ~[?:?]
2024-06-05 10:54:13 at org.opensearch.security.util.KeyUtils$1.run(KeyUtils.java:58) [opensearch-security-2.14.0.0.jar:2.14.0.0]
2024-06-05 10:54:13 at org.opensearch.security.util.KeyUtils$1.run(KeyUtils.java:45) [opensearch-security-2.14.0.0.jar:2.14.0.0]
2024-06-05 10:54:13 at java.base/java.security.AccessController.doPrivileged(AccessController.java:319) [?:?]
2024-06-05 10:54:13 at org.opensearch.security.util.KeyUtils.createJwtParserBuilderFromSigningKey(KeyUtils.java:45) [opensearch-security-2.14.0.0.jar:2.14.0.0]
2024-06-05 10:54:13 at com.amazon.dlic.auth.http.jwt.HTTPJwtAuthenticator.<init>(HTTPJwtAuthenticator.java:88) [opensearch-security-2.14.0.0.jar:2.14.0.0]
2024-06-05 10:54:13 at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62) ~[?:?]
2024-06-05 10:54:13 at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502) ~[?:?]
2024-06-05 10:54:13 at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:486) ~[?:?]

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)

Enhancement

Issues Resolved

Resolves: #4406

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• 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.

[derek-ho]

Minor comment

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.94%. Comparing base (703d40f) to head (f509b49).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4833      +/-   ##
==========================================
- Coverage   70.99%   70.94%   -0.05%     
==========================================
  Files         310      310              
  Lines       20938    20951      +13     
  Branches     3326     3331       +5     
==========================================
- Hits        14865    14864       -1     
- Misses       4325     4336      +11     
- Partials     1748     1751       +3     

Files with missing lines	Coverage Δ
...in/java/org/opensearch/security/util/KeyUtils.java	87.87% <100.00%> (+0.37%)	⬆️

... and 14 files with indirect coverage changes