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

projects/generate.bat

@@ -217,6 +217,7 @@ rem
       call :element %1 lib "curl_multibyte.c" %3
       call :element %1 lib "version_win32.c" %3
       call :element %1 lib "dynbuf.c" %3
+      call :element %1 lib "base64.c" %3
     ) else if "!var!" == "CURL_SRC_X_H_FILES" (
       call :element %1 lib "config-win32.h" %3
       call :element %1 lib "curl_setup.h" %3
@@ -228,6 +229,7 @@ rem
       call :element %1 lib "curl_multibyte.h" %3
       call :element %1 lib "version_win32.h" %3
       call :element %1 lib "dynbuf.h" %3
+      call :element %1 lib "curl_base64.h" %3
     ) else if "!var!" == "CURL_LIB_C_FILES" (
       for /f "delims=" %%c in ('dir /b ..\lib\*.c') do call :element %1 lib "%%c" %3
     ) else if "!var!" == "CURL_LIB_H_FILES" (