NIFI-16137 - Support externally-supplied client assertion in JWTBearerOAuth2AccessTokenProvider

[pvillard31]

Summary

NIFI-16137 - Support externally-supplied client assertion in JWTBearerOAuth2AccessTokenProvider

The idea is to support a scenario like:

External Identity Token Provider
        │ 
        ▼
JWTBearerOAuth2AccessTokenProvider 
        │  Token Endpoint = https://login.microsoftonline.com/<tenant>/oauth2/v2.0/token
        │  Client ID = Entra app's client ID
        │  Scope = api://<client-id>/.default   
        │  External Assertion Provider = ↑ references External Identity Token Provider
        ▼
AWSCredentialsProviderControllerService
        │  Assume Role ARN = the AWS IAM role trusting Entra
        │  Assume Role Session Name
        │  Assume Role STS Region
        │  OAuth2 Access Token Provider = ↑ references the JWTBearerOAuth2AccessTokenProvider
        ▼
ListS3 processor
        AWS Credentials Provider Service = ↑ references AWSCredentialsProviderControllerService

I tested the changes locally and can provide some screenshot if helpful.

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