Implementing JWKs

[shytikov]

Looks like a lot of changes, but I've tried to structure it logically:

• code from jwt.go was split – Config were moved to config.go and the main middleware call was moved to main.go;
• added jwks.go file which contains code from https://github.com/MicahParks/keyfunc
• rest are support files from keyfunc.

I haven't modified logic from the initial jwt.go. All configurations which were added are optional, and jwks will kick in only if KeySetUrl will be provided. That will replace the original keyfunc with the one that is able to work with jwks.

I believe I could get away with smaller changes if I would reference https://github.com/MicahParks/keyfunc directly. But that would bring additional unused legacy dependencies to this middleware and to the rest of the projects which will be using it, so I decided to copy files instead.

I have tested it on my JWKs server, and it works (on my machine :), including background updates of the keys.

I'm open to any suggestions that would make this PR better. Many thanks in advance!

[shytikov]

@ReneWerner87 could you please check is it LGTM?

I can continue covering with tests these pieces of code. But I am afraid that I will delay the code and I'm also forced into refactoring as I write the tests, so I wanted to put a comma somewhere.

[shytikov]

ping @ReneWerner87 :)

[ReneWerner87]

Jo, sure.. Will check it on the weekend) currently to much other stuff)

[shytikov]

Thank you! :)

[ReneWerner87]

@shytikov can you please correct the three comments of the automated test and the linter

[shytikov]

On it! @ReneWerner87 :)

[shytikov]

@ReneWerner87 Hopefully now everything shoudl be fine!