Assert UTransport implementations to be thread-safe

Most UTransport implementations will be implemented in a way that allows them to be invoked from multiple threads already. It therefore seems reasonable to let the UTransport trait extend Send + Sync to let client code rely on this fact.

src/rpc/rpcclient.rs

@@ -18,12 +18,12 @@ use crate::{CallOptions, RpcMapperError, UMessage, UPayload, UUri};
 pub type RpcClientResult = Result<UMessage, RpcMapperError>;
 
 /// `RpcClient` is an interface used by code generators for uProtocol services defined in `.proto` files such as
-/// the core uProtocol services found in [uProtocol Core API](https://github.com/eclipse-uprotocol/uprotocol-core-api).
+/// the core uProtocol services found in [uProtocol Core API](https://github.com/eclipse-uprotocol/up-spec/tree/main/up-core-api).
 ///
 /// The trait provides a clean contract for mapping a RPC requiest to a response. For more details please refer to the
-/// [RpcClient Specifications](https://github.com/eclipse-uprotocol/uprotocol-spec/blob/main/up-l2/README.adoc).
+/// [RpcClient Specifications](https://github.com/eclipse-uprotocol/up-spec/blob/main/up-l2/rpcclient.adoc).
 #[async_trait]
-pub trait RpcClient {
+pub trait RpcClient: Send + Sync {
     /// Invokes a method on a remote service asynchronously.
     ///
     /// This function is an API for clients to send an RPC request and receive a response.

src/utransport.rs

@@ -22,8 +22,7 @@ use crate::{UMessage, UStatus, UUri};
 ///
 /// Implementations of `UListener` contain the details for what should occur when a message is received
 ///
-/// For more information, please refer to
-/// [uProtocol Specification](https://github.com/eclipse-uprotocol/uprotocol-spec/blob/main/up-l1/README.adoc).
+/// For more information, please refer to [uProtocol Specification](https://github.com/eclipse-uprotocol/up-spec/blob/main/up-l1/README.adoc).
 ///
 /// # Examples
 ///
@@ -86,7 +85,7 @@ use crate::{UMessage, UStatus, UUri};
 /// }
 /// ```
 #[async_trait]
-pub trait UListener: 'static + Send + Sync {
+pub trait UListener: Send + Sync {
     /// Performs some action on receipt of a message
     ///
     /// # Parameters
@@ -128,9 +127,9 @@ pub trait UListener: 'static + Send + Sync {
 ///
 /// Implementations of [`UTransport`] contain the details for connecting to the underlying transport technology and
 /// sending [`UMessage`][crate::UMessage] using the configured technology. For more information, please refer to
-/// [uProtocol Specification](https://github.com/eclipse-uprotocol/uprotocol-spec/blob/main/up-l1/README.adoc).
+/// [uProtocol Specification](https://github.com/eclipse-uprotocol/up-spec/blob/main/up-l1/README.adoc).
 #[async_trait]
-pub trait UTransport {
+pub trait UTransport: Send + Sync {
     /// Sends a message using this transport's message exchange mechanism.
     ///
     /// # Arguments