Add access token support

[shawnhankim]

Issue:

• Add access token support #69

Description:

• Captured access_token in the NJS code.
• Stored access_token in the key/value store as same as the NGINX RP stores id_token and refresh_token.

[shawnhankim]

@route443 :

• Thanks for your review in detail for the PR.
• This PR is to simplify from the previous PR.
• For you to easily manage this repo to reduce any concerns from the enhancements based on the reviews on the PR, I have divided a big PR into small PRs, and this is one of PRs.
• Let me know if you have any questions.

[route443]

Hi Shawn. Thank you for your contribution to our nginx oidc project.
As I said in previous PRs, we are not using Conventional Commits for this project. So I wouldn't use this approach unnecessarily. Please change the commit name.

[shawnhankim]

Hi Shawn. Thank you for your contribution to our nginx oidc project. As I said in previous PRs, we are not using Conventional Commits for this project. So I wouldn't use this approach unnecessarily. Please change the commit name.

Hi Ivan! Thank you so much for your time to review the PR in detail. I have addressed your comments as well as the new commit name. Please let me know if there is anything need to be changed. Happy New Year!

[shawnhankim]

Thanks @route443 for your review. Revised them!