Port StorageThreadMsg to GenericChannel (#38932)

This change includes the following additions to GenericChannel:
- Add a GenericSend trait which is meant to replace the `IpcSend` trait
over time, as channels are migrated. For the time being this means, that
we often need to use `GenericSend::send()` to disambiguate from the
`IpcSend::send` function, until all usages of `IpcSend` have been
replaced.
- Add an OpaqueSender impl for GenericSender
- Add a profiled version of GenericChannel. The profiling is 1:1 the
same as for the existing profiled IPC channel, namely that only the
blocked time during `recv` is measured.


Testing: No functional changes, covered by existing tests
Part of #38912

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/shared/base/generic_channel.rs

@@ -15,6 +15,18 @@ use serde::de::VariantAccess;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use servo_config::opts;
 
+/// Abstraction of the ability to send a particular type of message cross-process.
+/// This can be used to ease the use of GenericSender sub-fields.
+pub trait GenericSend<T>
+where
+    T: serde::Serialize + for<'de> serde::Deserialize<'de>,
+{
+    /// send message T
+    fn send(&self, _: T) -> SendResult;
+    /// get underlying sender
+    fn sender(&self) -> GenericSender<T>;
+}
+
 /// A GenericSender that sends messages to a [GenericReceiver].
 ///
 /// The sender supports sending messages cross-process, if servo is run in multiprocess mode.