doc: Add description for pluggable RPCs

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## Adding external RPC methods

SPDK includes both in-tree modules as well as the ability to use external modules.  The in-tree modules include some python

scripts to ease the process of sending RPCs to in-tree modules.  External modules can utilize this same framework to add new RPC

methods as follows:

If PYTHONPATH doesn't include the location of the external RPC python script, it should be updated:

~~~

export PYTHONPATH=/home/user/plugin_example/

~~~

In provided location, create python module file (e.g. rpc_plugin.py) with new RPC methods.  The file should contain

spdk_rpc_plugin_initialize() method that will be called when the plugin is loaded to define new parsers for provided subparsers

argument that adds new RPC calls (subparsers.add_parser()).  The new parsers should use the client.call() method to call RPC

functions registered within the external module using the SPDK_RPC_REGISTER() macro.  Example:

~~~

from rpc.client import print_json

def example_create(client, num_blocks, block_size, name=None, uuid=None):

    """Construct an example block device.

    Args:

        num_blocks: size of block device in blocks

        block_size: block size of device; must be a power of 2 and at least 512

        name: name of block device (optional)

        uuid: UUID of block device (optional)

    Returns:

        Name of created block device.

    """

    params = {'num_blocks': num_blocks, 'block_size': block_size}

    if name:

        params['name'] = name

    if uuid:

        params['uuid'] = uuid

    return client.call('bdev_example_create', params)

def example_delete(client, name):

    """Delete example block device.

    Args:

        bdev_name: name of bdev to delete

    """

    params = {'name': name}

    return client.call('bdev_example_delete', params)

def spdk_rpc_plugin_initialize(subparsers):

    def bdev_example_create(args):

        num_blocks = (args.total_size * 1024 * 1024) // args.block_size

        print_json(example_create(args.client,

                                  num_blocks=int(num_blocks),

                                  block_size=args.block_size,

                                  name=args.name,

                                  uuid=args.uuid))

    p = subparsers.add_parser('bdev_example_create',

                              help='Create an example bdev')

    p.add_argument('-b', '--name', help="Name of the bdev")

    p.add_argument('-u', '--uuid', help="UUID of the bdev")

    p.add_argument(

        'total_size', help='Size of bdev in MB (float > 0)', type=float)

    p.add_argument('block_size', help='Block size for this bdev', type=int)

    p.set_defaults(func=bdev_example_create)

    def bdev_example_delete(args):

        example_delete(args.client,

                      name=args.name)

    p = subparsers.add_parser('bdev_example_delete',

                              help='Delete an example disk')

    p.add_argument('name', help='example bdev name')

    p.set_defaults(func=bdev_example_delete)

~~~

Finally, call the rpc.py script with '--plugin' parameter to provide above python module name:

~~~

./scripts/rpc.py --plugin rpc_plugin bdev_example_create 10 4096

~~~

# App Framework {#jsonrpc_components_app}

## spdk_kill_instance {#rpc_spdk_kill_instance}