zebra: add developer test functions for FPM code

zebra: add developer test functions for FPM code

Add test functions for the zebra code that interfaces with the
Forwarding Plane Manager. These functions can be invoked in a
development build via the recently-added 'invoke' command.

For example:

  # invoke zebra function zfpm_dt_benchmark_protobuf_encode 100000

Changes:

  * zebra/zebra_fpm_dt.c

    Add the following functions. Each function encodes or decodes a
    route in a particular FPM format a specified number of times.

    - zfpm_dt_benchmark_netlink_encode()
    - zfpm_dt_benchmark_protobuf_encode()
    - zfpm_dt_benchmark_protobuf_decode()

  * zebra/Makefile.am

    Compile zebra_fpm_dt when building a development build.

Signed-off-by: Avneesh Sachdev <avneesh@sproute.com>