Minor refactoring of OIDC DefaultTokenStateManager

[sberyozkin]

This PR is the first preparation PR for addressing #32109 (and #45093).

Let me clarify. OIDC DefaultTokenStateManager, by default, stores 3 tokens, ID, access, and refresh tokens as an encrypted cookie value:

1. Cookies are concatenated: ID token | access token | refresh token
2. This combined string is encrypted and it becomes a session cookie value.

But there are variations: storing the access and refresh tokens is optional , and also if token values are huge, each of these tokens can be represented by an individual cookie.

As far as #32109 is concerned, storing an additional property becomes very hard.

So this PR starts with restructuring a bit the way OIDC DefaultTokenStateManager currently deals with storing tokens in cookies and getting them out of cookies, and adds quite a few comments to make it clearer what is going on. Not a single piece of logic has changed.

I did it so that it could be easier to follow the changes in the planned second preparation PR, where tokens will be stored in JSON, which will make it much easier to deal with multiple tokens and some extra properties.

Also, FYI, @pedroigor @michalvavrik

[quarkus-bot]

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It should be safe to merge provided you have a look at the other checks in the summary.

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[gastaldi]

LGTM