Improve JSON marshal/unmarshal performance

[malud]

We are mostly using the Go build-in package to fulfil this tasks. But there are packages out there which are faster and have less memory consumption. We have dependencies which brings them already as peer dependency. We should take a look if we could improve our json handling.

Related sections:

jwk
jwks
oidc
jwt
oauth2
(ggf cty, in ctyjson -> prüfen)
json_body ?

go mod why -m github.com/mailru/easyjson

# github.com/mailru/easyjson
github.com/avenga/couper/handler/validation
github.com/getkin/kin-openapi/openapi3
github.com/go-openapi/jsonpointer
github.com/go-openapi/swag
github.com/mailru/easyjson/jlexer

According to the readme of easyjson we could improve the throughput to more than 100 MB/s in both directions.