[coap] add base class CoapSecureBase and ApplicationCoapSecure (#10945)

This commit refactors COAPS classes, renaming the `CoapSecure` class
as `CoapSecureBase`, which is the base class of `Tmf::SecureAgent`
and a newly added `ApplicationCoapSecure` class. This change
simplifies the code and class hierarchy and ensures that the
Application COAP secure related functions are only provided by
`ApplicationCoapSecure` and when the corresponding config feature
`OPENTHREAD_CONFIG_COAP_SECURE_API_ENABLE` is enabled.

src/core/thread/tmf.cpp

@@ -273,7 +273,7 @@ Message::Priority Agent::DscpToPriority(uint8_t aDscp)
 #if OPENTHREAD_CONFIG_SECURE_TRANSPORT_ENABLE
 
 SecureAgent::SecureAgent(Instance &aInstance)
-    : Coap::CoapSecure(aInstance, kNoLinkSecurity)
+    : Coap::CoapSecureBase(aInstance, kNoLinkSecurity)
 {
     SetResourceHandler(&HandleResource);
 }