fix(bigquery/storage/managedwriter): context refactoring

[shollyman]

This PR refactors how contexts are propagated/retained in the managedwriter package. Prior to multiplexing, each writer had a dedicated connection, and context could be shared between writer and the connection. Multiplexing decoupled these, and this PR addresses some issues with those changes.

Most notably, the library (incorrectly) used the context provided when instantiating a writer via NewManagedStream to derive the context for the connection pool, which would cause issues if said writer was closed before other writers in the pool completed/closed.

Similarly, (re)-opening a connection would reuse the same context, and certain failure scenarios (namely, server-close on an idle connection) could cause slow leak of grpc resources due to fully signalling disposal of the connection from the client side.

This CL corrects these issues by refactoring some of the key context lifecycles.

First, we use the context provided when instantiating the client (e.g. NewClient) as the root context when constructing connection pool and connections. It also preserves the desired property of making shutdown straightforward.

In the second, two key changes of note:

• This CL changes the internal signature for our connection opening logic to accept a context as part of the signature, as opposed to binding context once when define our opening closure. Now, opening uses the connection's current context.
• This CL now also cancels the previous context and recreates a new retained context and cancellation from its parent pool when we trigger reconnection logic. This also slightly alters the signature for our receiver goroutine to accept its context rather than referencing it from the connection itself, since the context lifecycles now differ.

Finally, this CL adds the requisite additional context expiry checks as we have potentially three contexts of note: the connection, the writer instance, and the request context (for a specific AppendRows request).

Fixes: #8232
Fixes: #8272