Add generic cross process callback mechanism (#38973)

One commonly encountered mechanism in servo is using ipc channels
together with the router, to register a custom callback to run in the
current process, when receiving a reply.
The new `GenericCallback` abstracts over this, and allows executing an
arbitrary callback in the process of the `GenericCallback` creator. In
multiprocess mode, this internally uses ipc channels and follows the
existing pattern.
In single process mode, we execute the callback directly, which avoids
one call to the router.
Executing the callback still incurs synchronization, since we need to
support cloning the abstraction, and the callback closure may be
`FnMut`. Future work could provide more optimized abstractions for
callbacks that don't have these requirements.

This PR allows applying #38782 again, which was previously reverted in
#38940 due to the lack of custom callback support.

See also the module documentation in `generic_channel/callback.rs`.

Testing: This PR adds unit tests. Also passes the manual testcase from
#38939

Part of #38912

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>
Signed-off-by: Jonathan Schwender <55576758+jschwe@users.noreply.github.com>
Co-authored-by: Josh Matthews <josh@joshmatthews.net>

components/shared/base/generic_channel.rs

@@ -15,6 +15,9 @@ use serde::de::VariantAccess;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use servo_config::opts;
 
+mod callback;
+pub use callback::GenericCallback;
+
 /// 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>