feat(xds): Add header mutations library

[sauravzg]

This PR sits on top of https://github.com/grpc/grpc-java/pull/12493, so only the last commit + any fixups need to be reviewed.

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.

• [] #12491 feat(xds): Update Envoy proto definitions and add ExtAuthz gRPC service
• [] #12492 feat(xds): Add configuration objects for ExtAuthz and GrpcService
• [] #12493 feat(xds): Implement request builder for external authorization
• [x] #12494 feat(xds): Add header mutations library
• [] TBD: PR_LINK feat(xds): Implement response handling for external authorization
• [] TBD: PR_LINK feat(xds): Add ExtAuthzClientInterceptor and related components
• [] TBD: PR_LINK feat(xds): Add ExtAuthzServerInterceptor and tests
• [] TBD: PR_LINK feat(xds): Add ExternalAuthorizationFilter