Implement Telegram provider

[holykol]

This PR adds Telegram as a login provider. The new flow uses deep-linking (see official example) to establish user identity.
Here are the steps:

1. User clicks link that opens up chat with login bot in local Telegram client
2. User presses 'Start' button which silently sends us some well-known value
3. The page magically logs user in (thanks to client-side polling)

TODO:

• Tests
• Downloading user avatars
• Auth request eviction

Closes #49

[coveralls]

Pull Request Test Coverage Report for Build 306194037

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 94.527%

---

Files with Coverage Reduction	New Missed Lines	%
auth.go	11	94.53%

Totals
Change from base Build 211748441:	0.03%
Covered Lines:	190
Relevant Lines:	201

---

💛 - Coveralls

[umputun]

Thx for the PR. The code is clean, easy to read, and probably does the right thing (I have not tried to run an example yet). I've added some (mostly minor) things to address. And another missing part - some documentation in the README about tg providers.

[holykol]

@umputun Thanks for the feedback! I've fixed things that you have mentioned and added some documentation for the feature (although I'm not sure if it is good enough).

[umputun]

I have tried to run _example but can't get it to work. Tried to follow the instruction, i.e. registred a new bot, got the token, passed it in, i.e. TELEGRAM_TOKEN="xxxxxx" go run main.go --dbg. After hitting start in tg client nothing good happens in the example app, all I see in log is 404 - GET - /auth/telegram/login?token=******** - [] - 404 (38) - 34.997µs

[holykol]

@umputun My bad. The bot username in _example was hardcoded, so you were probably sending start command to the wrong bot. Pushing a fix RN.

[umputun]

thx, worked

[umputun]

how about this issue?

as soon as user logged out, it starts to send those, constantly:

2020/10/13 17:32:48.863 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 26.34µs
2020/10/13 17:32:49.864 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 43.622µs
2020/10/13 17:32:50.863 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 35.893µs
2020/10/13 17:32:51.864 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 30.566µs
2020/10/13 17:32:52.865 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 76.823µs
2020/10/13 17:32:53.868 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 101.266µs
2020/10/13 17:32:55.872 [INFO]  {logger/logger.go:122 logger.(*Middleware).Handler.func1.1} GET - /auth/telegram/login?token=******** - [] - 404 (38) - 106.877µs

[holykol]

Fixed. The requests are now made only after link is clicked to avoid polluting logs.

[umputun]

LGTM, thx

[holykol]

Thanks for your time!