Preload private_key in JwtTokenSource

[LuckySting]

Loading cryptography keys from pem-encoded bytes string takes a lot of time (~250ms). Due to private_key is not changed during the life of JwtTokenSource it could be done only once, which would significantly speed up token issuing.

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• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
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What is the current behavior?

Currently the private key is loaded on each call of token() method, which takes ~250ms.

Issue Number: N/A

What is the new behavior?

Now the private key is loaded only once in class constructor, which leads to speed up of token() method from 250ms to 4ms.

Other information