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

docs/cmdline-opts/variable.d

@@ -0,0 +1,50 @@
+c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+SPDX-License-Identifier: curl
+Long: variable
+Arg: <[%]name=text/@file>
+Help: Set variable
+Category: curl
+Example: --variable name=smith $URL
+Added: 8.3.0
+See-also: config
+Multi: append
+---
+Set a variable with "name=content" or "name@file" (where "file" can be stdin
+if set to a single dash (-)). The name is a case sensitive identifier that
+must consist of no other letters than a-z, A-Z, 0-9 or underscore. The
+specified content is then associated with this identifier.
+
+The name must be unique within a command line invoke, setting the same
+variable name again will be ignored.
+
+The contents of a variable can be referenced in a later command line option
+when that option name is prefixed with "--expand-", and the name is used as
+"{{name}}" (without the quotes).
+
+--variable can import environment variables into the name space. Opt to either
+require the environment variable to be set or provide a default value for the
+variable in case it is not already set.
+
+--variable %name imports the variable called 'name' but exits with an error if
+that environment variable is not alreadty set. To provide a default value if
+the environment variable is not set, use --variable %name=content or
+--variable %name@content. Note that on some systems - but not all -
+environment variables are case insensitive.
+
+When expanding variables, curl supports a set of functions that can make the
+variable contents more convenient to use. You apply a function to a variable
+expansion by adding a colon and then list the desired functions in a
+comma-separted list that is evaluated in a left-to-right order. Variable
+content holding null bytes that are not encoded when expanded, will cause
+error.
+
+These are functions that can help you get the value inserted more
+conveniently.
+
+"trim" removes all leading and trailing white space.
+
+"json" outputs the content using JSON string quoting rules.
+
+"url" shows the content URL (percent) encoded.
+
+"b64" expands the variable base64 encoded