feature: Discord OAuth2 Provider

[mgkbadola]

Added login support for Discord. If this is approved and merged, I would like to add it's its support to Remark42 (love that service).

A request: Since I am currently participating in Hacktoberfest, if this PR is to be merged, kindly add the label hacktoberfest-accepted to it. Many thanks!

[umputun]

Thanks. I don't see anything wrong with this PR. @paskal - can you please take a look and double-check?

[umputun]

@mgkbadola - pls fix the linter issue, look like a public method without a comment

[mgkbadola]

@mgkbadola - pls fix the linter issue, look like a public method without a comment

on it 👍🏽 done

[paskal]

Thanks, will check and test shortly. Could you please add it to the v2 as well?

[coveralls]

Pull Request Test Coverage Report for Build 11195676729

Details

• 21 of 23 (91.3%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 83.344%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
provider/providers.go	19	21	90.48%
<!--	Total:	21	23	91.3%	-->

Totals
Change from base Build 11078013402:	0.06%
Covered Lines:	2612
Relevant Lines:	3134

---

💛 - Coveralls

[paskal]

According to the rules:

For your pull requests to be accepted, the request must either be merged, have the Hacktoberfest-accepted label, or have an overall approving review on GitHub.

I think this PR will be counted once merged.

[badolamgk]

Hey/Privyet @paskal I have done the required changes. Thanks for reviewing and providing reference for Remark42.

[badolamgk]

Hey @umputun

Let me know if there are any other changes required on my end. Cheers.

[badolamgk]

For the below command,

go test -timeout=60s -v -race -p 1 -covermode=atomic ./...

Test cases passing on my workstation, attaching logs here.