tool: add "variable" support

Add support for command line variables. Set variables with --variable
name=content or --variable name@file (where "file" can be stdin if set
to a single dash (-)).

Variable content is expanded in option parameters using "{{name}}"
(without the quotes) if the option name is prefixed with
"--expand-". This gets the contents of the variable "name" inserted, or
a blank if the name does not exist as a variable. Insert "{{" verbatim
in the string by prefixing it with a backslash, like "\\{{".

Import an environment variable with --variable %name. It makes curl exit
with an error if the environment variable is not set. It can also rather
get a default value if the variable does not exist, using =content or
@file like shown above.

Example: get the USER environment variable into the URL:

 --variable %USER
 --expand-url = "https://example.com/api/{{USER}}/method"

When expanding variables, curl supports a set of functions that can make
the variable contents more convenient to use. It can trim leading and
trailing white space with "trim", output the contents as a JSON quoted
string with "json", URL encode it with "url" and base 64 encode it with
"b64". To apply functions to a variable expansion, add them colon
separated to the right side of the variable. They are then performed in
a left to right order.

Example: get the contents of a file called $HOME/.secret into a variable
called "fix". Make sure that the content is trimmed and percent-encoded
sent as POST data:

  --variable %HOME=/home/default
  --expand-variable fix@{{HOME}}/.secret
  --expand-data "{{fix:trim:url}}"
  https://example.com/

Documented. Many new test cases.

Co-brainstormed-by: Emanuele Torre
Assisted-by: Jat Satiro
Closes #11346

lib/base64.c

@@ -32,13 +32,14 @@
   !defined(CURL_DISABLE_POP3) || \
   !defined(CURL_DISABLE_IMAP) || \
   !defined(CURL_DISABLE_DOH) || defined(USE_SSL)
-
-#include "urldata.h" /* for the Curl_easy definition */
+#include "curl/curl.h"
 #include "warnless.h"
 #include "curl_base64.h"
 
 /* The last 2 #include files should be in this order */
+#ifdef BUILDING_LIBCURL
 #include "curl_memory.h"
+#endif
 #include "memdebug.h"
 
 /* ---- Base64 Encoding/Decoding Table --- */

lib/curl_base64.h

@@ -24,11 +24,18 @@
  *
  ***************************************************************************/
 
+#ifndef BUILDING_LIBCURL
+/* this renames functions so that the tool code can use the same code
+   without getting symbol collisions */
+#define Curl_base64_encode(a,b,c,d) curlx_base64_encode(a,b,c,d)
+#define Curl_base64url_encode(a,b,c,d) curlx_base64url_encode(a,b,c,d)
+#define Curl_base64_decode(a,b,c) curlx_base64_decode(a,b,c)
+#endif
+
 CURLcode Curl_base64_encode(const char *inputbuff, size_t insize,
                             char **outptr, size_t *outlen);
 CURLcode Curl_base64url_encode(const char *inputbuff, size_t insize,
                                char **outptr, size_t *outlen);
 CURLcode Curl_base64_decode(const char *src,
                             unsigned char **outptr, size_t *outlen);
-
 #endif /* HEADER_CURL_BASE64_H */