Add ProviderMetadata::discover_with_options and "common" Entra example

[phy1729]

#179 handles the case for an Entra application that allows signins from a single tenant. For Entra applications that allow signins from personal and enterprise accounts, the https://login.microsoftonline.com/common/v2.0 issuer is used which returns a metadata document with issuer set to https://login.microsoftonline.com/{tenantid}/v2.0.

This PR adds ProviderMetadata::discover_with_options and ProviderMetadata::discover_async_with_options to disable the issuer validation required by the OIDC spec. I think this is more discoverable than the proposed route in #122 of wrapping http_client to munge the response.

In the example require_issuer_match is disabled on the IdTokenVerifier because the ID token has the iss claim set with the specific tenant ID of the account which is not a static value.

[maxammann]

thank you! @ramosbugs this seems required for typical SSO setups with MS

[maxammann]

A workaround is to do:

        let client = CoreClient::new(
            ClientId::new(client_id.to_string()),
            issuer_url,
            JsonWebKeySet::fetch_async(
                &JsonWebKeySetUrl::new("https://login.microsoftonline.com/common/discovery/v2.0/keys".to_string())?,
                &http_client,
            )
            .await?,
        )
        .set_auth_uri(oauth2::AuthUrl::new(
            "https://login.microsoftonline.com/common/oauth2/v2.0/authorize".to_string(),
        )?)
        .set_token_uri(oauth2::TokenUrl::new(
            "https://login.microsoftonline.com/common/oauth2/v2.0/token".to_string(),
        )?)