[Enhancement] Set up a Util class for OBO Authenticator for key checks and endpoints checks

[RyanL1997]

Description

Transferring some of the discussion from here, it is reasonable to setup a public field in codebase to holding some of the common logic for OBO authentication, such as the configuration checks.

Related issue/PR

• Related [META] On-Behalf-Of Authentication #2573
• Relate OnBehalfOf authenticator and token generator #3179 (need the merge of this)

Potential solutions

Potential place 1: ~/security/src/test/java/org/opensearch/security/authtoken

• reason: it is related to the OnBehalfOf feature, so that we can setup an util class in here.

**Potential place 2: ~/security/src/main/java/org/opensearch/security/util

• reason: Just like its name it is the util directory. (Recommended)

Exit Criteria

Set up the ultil class to place the reusable logic for checking endpoint calls, checking keys.

[peternied]

[Triage] It isn't clear what the expected outcome since the source isn't as obvious. Maybe a pull request would answer these questions?

[stephen-crawford]

[Triage] Hi @RyanL1997 following up on this for additional details. Thank you.

[peternied]

@peternied Can you do a quick review of this work