Add `--filter enabled=true` for `docker plugin ls`

This fix adds `--filter enabled=true` to `docker plugin ls`,
as was specified in 28624.

The related API and docs has been updated.

An integration test has been added.

This fix fixes 28624.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/commandline/plugin_ls.md

@@ -24,6 +24,7 @@ Aliases:
   ls, list
 
 Options:
+  -f, --filter filter   Provide filter values (e.g. 'enabled=true')
       --format string   Pretty-print plugins using a Go template
       --help            Print usage
       --no-trunc        Don't truncate output
@@ -32,6 +33,8 @@ Options:
 
 Lists all the plugins that are currently installed. You can install plugins
 using the [`docker plugin install`](plugin_install.md) command.
+You can also filter using the `-f` or `--filter` flag.
+Refer to the [filtering](#filtering) section for more information about available filter options.
 
 Example output:
 
@@ -42,6 +45,20 @@ ID                  NAME                             TAG                 DESCRIP
 69553ca1d123        tiborvass/sample-volume-plugin   latest              A test plugin for Docker   true
 ```
 
+## Filtering
+
+The filtering flag (`-f` or `--filter`) format is of "key=value". If there is more
+than one filter, then pass multiple flags (e.g., `--filter "foo=bar" --filter "bif=baz"`)
+
+The currently supported filters are:
+
+* enabled (boolean - true or false, 0 or 1)
+
+### enabled
+
+The `enabled` filter matches on plugins enabled or disabled.
+
+
 ## Formatting
 
 The formatting options (`--format`) pretty-prints plugins output
@@ -68,6 +85,7 @@ $ docker plugin ls --format "{{.ID}}: {{.Name}}"
 4be01827a72e: tiborvass/no-remove
 ```
 
+
 ## Related information
 
 * [plugin create](plugin_create.md)