Rework JwtProxyConfigBuilder to use Objects instead of formatting string config template

[sleshchenko]

Description

As a sequel of the following discussion #10252 (comment)

It is needed to rework JwtProxyConfigBuilder to use Objects instead of formatting string config template.

So, it includes:

• creation JWTProxy config model objects as Java classes. Here is an actual state of jwtproxy config model in go https://github.com/eclipse-che-staging/jwtproxy/blob/master/config/config.go;
• reworking JwtProxyConfigBuilder to create the corresponding JWTProxyConfig object and serialize it to YAML format instead of formatting string config template;

[sleshchenko]

Also, I think it makes sense not make configurable jwtproxy configuration properties that now are hardcoded. Like, verifier.max_skew, verifier.max_ttl, jwt proxy image and maybe other.