feat(api-core): add ClientInterceptor and apply_interceptors helper

[chalmerlowe]

Problem

Generated client libraries currently lack a standardized, centralized helper in google-api-core to apply client interceptors to a gRPC channel. Without a shared helper, each client library would need duplicate channel interceptor wrapping logic.

Solution

This change introduces interceptor utilities in google.api_core.grpc_helpers:

• ClientInterceptor: A generic type alias representing client-side gRPC interceptors across unary and streaming modes.
• apply_interceptors: A utility function that applies an optional list of interceptors to a gRPC channel in a single call via grpc.intercept_channel(channel, *interceptors). If no interceptors are provided, it returns the original channel unmodified.
• We include comprehensive unit tests covering passthrough behavior and single/multiple interceptor application.

Notes for Reviewers

• Passing all interceptors unpacked into grpc.intercept_channel(channel, *interceptors) delegates directly to gRPC's native variadic API and ensures standard execution order where the first interceptor in the sequence executes first on outbound requests.

[gemini-code-assist]

Code Review

This pull request introduces the apply_interceptors helper function to sequentially apply a list of client interceptors to a gRPC channel, along with comprehensive unit tests verifying its behavior. The reviewer feedback correctly points out that applying interceptors sequentially in a loop introduces unnecessary nesting overhead and reverses the standard gRPC execution order. To resolve this, the reviewer suggests unpacking the interceptors directly into a single grpc.intercept_channel call and updating the corresponding execution order test assertion.

[daniel-sanche]

In your other PR, I suggested accepting Callable wrappers here as an alternate interceptor type, to support the otel interceptor.

That would make this into something like:

modified_channel = channel
for interceptor in interceptors or []:
    if isinstance(interceptor, ClientInterceptor):
        modified_channel = grpc.intercept_channel(channel, interceptor)
    else:
        modified_channel = interceptor(modified_channel)
return modified channel

Let me know if you think that could work