Update go-cty to pull in more stdlib funcs.

I needed "split" specifically so I can do something like:

```hcl
variable PLATFORMS {
  default = "linux/amd64"
}

target foo {
  platforms = split(",", "${PLATFORMS}")
  # other stuff
}
```

Where the existing "csvdecode" does not work for this because it parses
the string into a list of objects instead of a list of strings.

I went ahead and just added all the available new functions.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

vendor/github.com/zclconf/go-cty/cty/function/stdlib/datetime.go

@@ -203,6 +203,33 @@ var FormatDateFunc = function.New(&function.Spec{
 	},
 })
 
+// TimeAddFunc is a function that adds a duration to a timestamp, returning a new timestamp.
+var TimeAddFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "timestamp",
+			Type: cty.String,
+		},
+		{
+			Name: "duration",
+			Type: cty.String,
+		},
+	},
+	Type: function.StaticReturnType(cty.String),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		ts, err := parseTimestamp(args[0].AsString())
+		if err != nil {
+			return cty.UnknownVal(cty.String), err
+		}
+		duration, err := time.ParseDuration(args[1].AsString())
+		if err != nil {
+			return cty.UnknownVal(cty.String), err
+		}
+
+		return cty.StringVal(ts.Add(duration).Format(time.RFC3339)), nil
+	},
+})
+
 // FormatDate reformats a timestamp given in RFC3339 syntax into another time
 // syntax defined by a given format string.
 //
@@ -383,3 +410,20 @@ func splitDateFormat(data []byte, atEOF bool) (advance int, token []byte, err er
 func startsDateFormatVerb(b byte) bool {
 	return (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z')
 }
+
+// TimeAdd adds a duration to a timestamp, returning a new timestamp.
+//
+// In the HCL language, timestamps are conventionally represented as
+// strings using RFC 3339 "Date and Time format" syntax. Timeadd requires
+// the timestamp argument to be a string conforming to this syntax.
+//
+// `duration` is a string representation of a time difference, consisting of
+// sequences of number and unit pairs, like `"1.5h"` or `1h30m`. The accepted
+// units are `ns`, `us` (or `µs`), `"ms"`, `"s"`, `"m"`, and `"h"`. The first
+// number may be negative to indicate a negative duration, like `"-2h5m"`.
+//
+// The result is a string, also in RFC 3339 format, representing the result
+// of adding the given direction to the given timestamp.
+func TimeAdd(timestamp cty.Value, duration cty.Value) (cty.Value, error) {
+	return TimeAddFunc.Call([]cty.Value{timestamp, duration})
+}