Refactor oidc-authservice by adding 4 new packages

[athamark]

Goal

In this PR, we refactor our oidc-authservice codebase. There are two upstream PRs that correlate with this effort:

1. Improvements to authenticator components #76 by @kellyma2, this PR not only reorganizes the directory/packages structure of our repo, but also applies a series of changes to isolate specific dependencies (for example coreos/go-oidc) so that they are called only within the respective packages. This is a good approach, one problem with simply proceeding with reviewing this PR is that it is outdated (over 1 year) and there are many changes which are not included. Moreover, we want a different package structure than the one proposed in the PR. We did however cherry-pick one commit that complied with our target.
2. Introduce pkg/common for common functionality across the code #34, this is an old PR from @yanniszark and @apyrgio which proposes the introduction of pkg/common.

We examined these two efforts and concluded to a solution that hopefully combines the best aspects of both. Undoubtedly, we deviated from the above approaches wherever necessary.

Useful Material

Here is a codebase refactoring in Golang article that provides the necessary pronciples that we need to considerate when refactoring our code: https://go.dev/talks/2016/refactor.article. One important thing that we need to keep in mind is that the package structure we will implement needs to avoid any circular dependencies.

The functions, variables, structs, struct fields, interfaces and constant values names that we want to export from every one of the new packages, need to start with a capital otherwise they will not be accessible/visible to the caller. Source: https://go.dev/tour/basics/3

Exported names
In Go, a name is exported if it begins with a capital letter.
[...]
When importing a package, you can refer only to its exported names. Any "unexported" names are not accessible from outside the package.

Proposed Implementation

The goal is to introduce 4 new packages:

1. common
2. oidc
3. sessions
4. authenticators

We need to introduce these packages in the order described above.

Local package dependencies

1. main package will import authenticators, session, oidc and common
2. authenticators package will import sessions, oidc and common
3. sessions package will import oidc and common
4. oidc package will import common

To ensure that no circular dependency gets introduced we will have to transfer some functions to different files.

File structure

Here is what each directory package will include, in parenthesis I expose the old filename:

/authenticators
  |- authenticator.go
  |- idtoken.go (authenticator_idtoken.go)
  |- jwt_test.go (authenticator_jwt_test.go)
  |- jwt.go (authenticator_jwt.go)
  |- kubernetes.go (authenticator_kubernetes.go)
  |- opaque_test.go (authenticator_opaque_test.go)
  |- opaque.go (authenticator_opaque.go)
  |- session.go (authenticator_session.go)
/common
  |- errors.go
  |- settings_test.go
  |- settings.go
  |- transformer_userid_test.go
  |- transformer_userid.go
  |- util_test.go
  |- util.go
/oidc
  |- oidc_test.go
  |- oidc.go
  |- revoke.go
/sessions
  |- boltdb.go (session_store_boltdb.go)
  |- redis.go (session_store_redis.go)
  |- session.go
  |- state.go

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Note to the reviewer

Since the #103 PR is not yet merged, we worked on top of it.