This commit introduces the `CheckRequestBuilder` library, which is responsible for constructing the `CheckRequest` message sent to the external authorization service.

The `CheckRequestBuilder` gathers information from various sources, including:
- `ServerCall` attributes (local and remote addresses, SSL session).
- `MethodDescriptor` (full method name).
- Request headers.

It uses this information to populate the `AttributeContext` of the `CheckRequest` message, which provides the authorization service with the necessary context to make an authorization decision.

This commit also introduces the `ExtAuthzCertificateProvider`, a helper class for extracting certificate information, such as the principal and PEM-encoded certificate.

Unit tests for the new components are also included.

This commit introduces a library for handling header mutations as specified by the xDS protocol. This library provides the core functionality for modifying request and response headers based on a set of rules.

The main components of this library are:
- `HeaderMutator`: Applies header mutations to `Metadata` objects.
- `HeaderMutationFilter`: Filters header mutations based on a set of configurable rules, such as disallowing mutations of system headers.
- `HeaderMutations`: A value class that represents the set of mutations to be applied to request and response headers.
- `HeaderMutationDisallowedException`: An exception that is thrown when a disallowed header mutation is attempted.

This commit also includes comprehensive unit tests for the new library.

This commit introduces the `CheckResponseHandler` and `AuthzResponse` classes, which are responsible for processing responses from the external authorization service.

The `CheckResponseHandler` parses the `CheckResponse` protobuf, determines whether the request should be allowed or denied, and applies any header mutations specified in the response. It handles both `OkHttpResponse` and `DeniedHttpResponse` messages.

The `AuthzResponse` class is a value object that represents the outcome of the authorization check, encapsulating the decision (allow or deny), the status to be returned to the client (for deny decisions), and any header mutations.

This commit also includes unit tests for the new components.

This commit introduces the client-side implementation of the external authorization filter.

The main component is the `ExtAuthzClientInterceptor`, which intercepts outgoing RPCs and performs external authorization checks. It uses a `BufferingAuthzClientCall` to buffer the outgoing RPC until the authorization decision is received from the authorization service.

The following new classes are introduced:
- `ExtAuthzClientInterceptor`: The main client interceptor for external authorization.
- `BufferingAuthzClientCall`: A `ClientCall` implementation that buffers requests until an authorization decision is made.
- `CallBuffer`: A helper class for `BufferingAuthzClientCall` to manage the buffered calls.
- `FailingClientCall`: A utility `ClientCall` that immediately fails, used when the filter is disabled and configured to deny calls.

This commit also includes comprehensive unit and integration tests for the new components.