lib: add trace support for client reads and writes

- add `CURL_TRC_READ()` and `CURL_TRC_WRITE()`
- use in generic client writers and readers, as well
  as http headers, chunking and websockets

Closes #13223

docs/libcurl/curl_global_trace.md

@@ -68,7 +68,7 @@ that.
 
 ## `tcp`
 
-Tracing of TCP socket handling: connect, reads, writes.
+Tracing of TCP socket handling: connect, sends, receives.
 
 ## `ssl`
 
@@ -96,6 +96,14 @@ trace.
 
 Tracing of DNS-over-HTTP operations to resolve hostnames.
 
+## `read`
+
+Traces reading of upload data from the application in order to send it to the server.
+
+## `write`
+
+Traces writing of download data, received from the server, to the application.
+
 # EXAMPLE
 
 ~~~c