public String asHeaderValue() {
        return CaseUtil.firstUpperAllLowerCase(authorizationType) +
               AUTHORIZATION_SEPARATOR + composeAuthorizationString();
    }
Fetches client authorization token or client credentials from the supplier and composes client
{@code Authorization} header value,
as per <a href="https://datatracker.ietf.org/doc/html/rfc6749#section-2.3">[RFC6749], Section 2.3</a>:
<pre>{@code
Authorization: Basic czZCaGRSa3F0Mzo3RmpmcDBaQnIxS3REUmJuZlZkbUl3
}</pre>.

@return encoded client {@code Authorization} header value.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/ClientAuthorization.java