Add support for API key provider

[nirinchev]

Adds support for API key provider and authentication.

• Adds support for User.apiKeys. This is a client that exposes functionality for managing user API keys.
• Adds support for Credentials.apiKey and Credentials.serverApiKey.
• Exposes User.accessToken and User.refreshToken.

I had to downgrade Core due to realm/realm-core#5890.

Fixes #393
Fixes #385
Fixes #386
Fixes #387
Fixes #388
Fixes #389
Fixes #390
Fixes #435
Fixes #433
Fixes #432
Fixes #394

[coveralls]

Pull Request Test Coverage Report for Build 3161647487

• 123 of 129 (95.35%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.175%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/src/app.dart	2	4	50.0%
lib/src/native/realm_core.dart	90	92	97.83%
lib/src/user.dart	29	31	93.55%

Totals
Change from base Build 3161270558:	0%
Covered Lines:	1910
Relevant Lines:	2191

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💛 - Coveralls

[nielsenko]

LGTM

[desistefanova]

Added some comments.
Changelog is missing.

[desistefanova]

Some awaits were missing.

[desistefanova]

I didn't find any test for serverApiKey. It would be nice to have. I am not sure if it is possible to generate any key. Maybe we can write a manual test, just to have it.

[nirinchev]

@desistefanova I've decided not to expose server API key since it's the same as user api key - see @ironage's comment here: realm/realm-core#5914 (comment). LMK if that makes sense to you. I can confirm that the server api key tests pass with the api key provider, so seems superficial indeed.

@blagoev I merged this with master, so it's no longer touching the native implementation.

[desistefanova]

@desistefanova I've decided not to expose server API key since it's the same as user api key - see @ironage's comment here: realm/realm-core#5914 (comment). LMK if that makes sense to you. I can confirm that the server api key tests pass with the api key provider, so seems superficial indeed.

@blagoev I merged this with master, so it's no longer touching the native implementation.

@nirinchev But it is possible in the Atlas web UI to create API key that doesn't belong to any user. How the developers will be able to use this key? See the documentation
https://www.mongodb.com/docs/atlas/app-services/authentication/api-key/

[nirinchev]

They can use Credentials.apiKey - apparently there's no difference in the two credential types. You can see the last couple of tests in user_test.dart are logging in a user with a server API key.

[blagoev]

Not sure about the expectLater calls. Should we always await the action or that is not mandatory?