zebra: add new CLI to manipulate srv6-locator (step2)

zebra: add new CLI to manipulate srv6-locator (step2)

This commit is a part of #5853 works that add new clis to
configure SRv6 locator and its show commands.
Following clis are added on this commit.

vtysh -c 'conf te' \
  -c 'segment-routing' \
  -c ' srv6' \
  -c '  locators' \
  -c '   locator LOC1' \
  -c '    prefix A::/64'

- "show segment-routing srv6 sid [json]"
- "show segment-routing srv6 locator [json]"
- "show segment-routing srv6 locator NAME detail [json]"
- "show runnning-config" (make it to print srv6 configuration)

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>