internal/mcp: clean up handling of content

Add support for all content types, and formalize the conversion of
content to and from the wire format (as a discriminated union).

Change-Id: I93678ce98c02176a524d2c82425fb1267ad68bf0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>

internal/util/util.go

@@ -10,6 +10,15 @@ import (
 	"slices"
 )
 
+// Apply returns a new slice resulting from applying f to each element of x.
+func Apply[S ~[]E, E, F any](x S, f func(E) F) []F {
+	y := make([]F, len(x))
+	for i, e := range x {
+		y[i] = f(e)
+	}
+	return y
+}
+
 // Helpers below are copied from gopls' moremaps package.
 
 // sorted returns an iterator over the entries of m in key order.