completion: improve detection for flags that accept multiple values (#2210)

The completion code attempts to detect whether a flag can be specified
more than once, and therefore should provide completion even if already
set.

Currently, this code depends on conventions used in the pflag package,
which uses an "Array" or "Slice" suffix or for some types a "stringTo"
prefix.

Cobra allows custom value types to be used, which may not use the same
convention for naming, and therefore currently aren't detected to allow
multiple values.

The pflag module defines a [SliceValue] interface, which is implemented
by the Slice and Array value types it provides (unfortunately, it's not
currently implemented by the "stringTo" values).

This patch adds a reduced interface based on the [SliceValue] interface
mentioned above to allow detecting Value-types that accept multiple values.
Custom types can implement this interface to make completion work for
those values.

I deliberately used a reduced interface to keep the requirements for this
detection as low as possible, without enforcing the other methods defined
in the interface (Append, Replace) which may not apply to all custom types.

Future improvements can likely still be made, considering either implementing
the SliceValue interface for the "stringTo" values or defining a separate
"MapValue" interface for those types.

Possibly providing the reduced interface as part of the pflag module and
to export it.

[SliceValue]: https://github.com/spf13/pflag/blob/d5e0c0615acee7028e1e2740a11102313be88de1/flag.go#L193-L203

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>
Co-authored-by: Marc Khouzam <marc.khouzam@gmail.com>