Deploy full oauth functionality

[TotallyNotChase]

Goal of the pull request:

• [ ] Documentation
• [ ] Bugfix
• [x] Feature (New!)
• [ ] Enhancement

Description

Sets up oauth for both application access tokens (client credentials) and user access tokens (authorization grant). Closes #93

Changelog

• Add getUserAuthorizationUrl, getUserTokenByCode, and getUserTokenByRefresh for getting user access tokens
• Add setUserAccessToken and setAppAccessToken, for setting user token and app access token respectively. These are designed to take the ebay api response as a parameter and set them accordingly
• Add oauthEndpoint for both SANDBOX and PROD to constants, this is used for generating the user consent url
• Rename src/common-utils/index/getAccessToken -> src/index/getAccessToken, with a few minor changes. Namely, explicitly set this.options.data instead of using this.options.body. This is done to be streamlined across all the token getter functions. getUserTokenByCode must use a custom this.options.data as it requires a code.

Note: This kind of makes this.options.body.grant_type useless, because each grant type has its own function but this will not break front end user functionality. In time, I suggest the options.body be completely removed and the scopes be passed in to their respective token getters as a parameter, hence achieving Single Responsibility Principle.

• Rename src/common-utils/index/setAccessToken -> src/index/setAppAccessToken, with a few minor changes. The parameter is now the full response returned by the ebay server and instead of being stored in options.access_token, it's now stored in options.appAccessToken
• Rename this.options.access_token -> this.options.appAccessToken
• Add getUserToken demo

Things to keep in mind during dev

• The endpoints that require an application access token (client credentials) should use this.options.appAccessToken (previously this.options.access_token)
• The endpoints that require a user access token (authorization grant) should use this.options.userAccessToken (previously unavailable)
• The refresh token for user access token is stored in this.options.refreshToken
• When a user is redirected after granting consent in the url returned by getUserAuthorizationUrl, the code present in the redirected url is url encoded. It must be url decoded before being passed into getUserTokenByCode

[TotallyNotChase]

@pajaydev did you get a chance at reviewing the changes?

[pajaydev]

@TotallyNotChase Sorry for the delay, I was caught up with some other work. I gave a small comment, kindly let me know if you are busy I will merge this and do it as part of cleanup. Thanks again for your patience 👍

[TotallyNotChase]

@pajaydev The changes are now migrated to src/credentials.js

[TotallyNotChase]

@pajaydev sounds good! thanks!

[emporioreale]

How to properly use that feature? Please post the working code to get an user access token by a refresh token. The demo code can't work because the refresh token parameter of the getUserTokenByRefresh method is missing.