refactor(jans-cedarling): make JwtService process tokens one at a time

[rmarinn]

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Description

• Development target issue

Target issue: refactor(jans-cedarling): make tokens optional #10408

• Implementation Details

since we need to make tokens optional for #10408, the current interface for the token validation should be updated so that all tokens would be optional:

Current implementation:

    pub fn process_tokens<'a, A, I, U>(
        &'a self,
        access_token: &'a str,
        id_token: &'a str,
        userinfo_token: Option<&'a str>,
    ) -> Result<ProcessTokensResult<'a, A, I, U>, JwtProcessingError>
    where
        A: Option<>,
        I: DeserializeOwned,
        U: DeserializeOwned,
    {
        // ...
    }

Proposed implementation:
Something like this would probably easier to use for the authz module. ProcessTokensResult should probably return an Option<TrustedIssuer> to find the trusted issuer need for the mapping.

enum UnprocessedJwt {
    Access(String),
    Id(String),
    Userinfo(String),
}

struct ProcessedJwt;

pub fn process_token(
    &'a self,
    token: UnprocessedJwt ,
) -> Result<ProcessedJwt, JwtProcessingError>
{
    // ....
}

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Prepare

• Read contribution guidelines
• Read license information

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Identified code changes

• implement a single function for authz to call for validating a token.

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Test cases and code coverage

• Write unit test to cover added/changed code
• Update integration tests to cover added/changed code NA

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Document the changes

• task for updating user guides if needed NA
• task for updating installation and configuration guides if needed NA
• task for updating developer documentation if needed NA
• task for updating technical documentation if needed NA

[rmarinn]

will be implemented with #10408.