Allow using custom headers in the Athenz module

[ikhoon]

The header names used to send an Athenz token is fixed to:

armeria/athenz/src/main/java/com/linecorp/armeria/common/athenz/TokenType.java

Lines 33 to 42 in d7fde99

	ACCESS_TOKEN(HttpHeaderNames.AUTHORIZATION, false, "Bearer"),
	/**
	* Athenz role token. {@code "Athenz-Role-Auth"} is used as the header name for this token type.
	*/
	ATHENZ_ROLE_TOKEN(HttpHeaderNames.ATHENZ_ROLE_AUTH, true, null),
	/**
	* The legacy Athenz role token used by Yahoo. {@code "Yahoo-Role-Auth"} is used as the
	* header name for this token type.
	*/
	YAHOO_ROLE_TOKEN(HttpHeaderNames.YAHOO_ROLE_AUTH, true, null);

I received internal feedback from LY that they prefer to use a custom header name rather than the standard Athenz header names.