lib: add new structures for srv6-locator (step2)

lib: add new structures for srv6-locator (step2)

This commit is a part of #5853 works that add new structures for
SRv6-locator. This structure will be used by zebra and another
routing daemon and its ZAPI messaging to manage SRv6-locator.
Encoder/decoder for ZAPI stream is also added by this commit.

Real configuration mechanism isn't implemented at this commit.
later commit add real configure implementation. This commit add only
SRv6-locator's structures and misc functions.

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