fix #457 by reusing OAuth for http bearer authentication

Created by: davidwcarlson

Fix proposed for Issue https://github.com/OpenAPITools/openapi-generator/issues/457 Similar to Issue https://github.com/OpenAPITools/openapi-generator/issues/1446 for typescript, Issue https://github.com/OpenAPITools/openapi-generator/issues/1577 for python

Specs defined as follows currently generate HttpBasicAuth and send Authorization: Basic [base64Encode(username + ":" + password)]

components:
  securitySchemes:
    bearer:
      type: http
      scheme: bearer

This PR will generate an OAuth header, which will send a Authorization: Bearer [accessToken] This is a smaller, less-impactful change than introducing an HttpBearerAuth object, but this change doesn't support scheme values other than "bearer"

This PR is an alternative to PR https://github.com/OpenAPITools/openapi-generator/pull/1930

Copying the Java technical committee as this is a change targeted to Java. @bbdouglas @JFCote @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger

PR checklist

• [ X] Read the contribution guidelines.
• [ X] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [X ] Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• [ X] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)