Service account credentials (key)

[MikhailNazarov]

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Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe): add dependency to jsonwebtoken for jwt and chrono for parsing token expiration

What is the current behavior?

There is no credential provider for service account key json file.
reqwest not allow https requests saying that only http is allowed

What is the new behavior?

Implemented ServiceAccountCredentials
Also exampl auth-ycloud-serviceaccount was provided for usage demonstation and debugging.

changed reqwest feature from rustls to rustls-tls fixed issue of sending https requests.

Other information

Example:

use ydb::ServiceAccountCredentials;
let cred = ServiceAccountCredentials::from_env()?;

or

let json = "....";
let cred = ServiceAccountCredentials::from_json(json)?;

or

let cred = ServiceAccountCredentials::new("service_account_id", "key_id", "private_key");

or

let cred = ServiceAccountCredentials::from_file("/path/to/file")?;

[rekby]

Hello @MikhailNazarov.

Thanks for great work!

I have few comments, for do the PR even better.