zebra: Add support for SRv6 SID blocks

zebra: Add support for SRv6 SID blocks

An SRv6 block is an IPv6 prefix from which SIDs are allocated. This
commit adds support for SRv6 SID blocks. Specifically, it adds a data
structure to store information about an SRv6 block (e.g., its occupancy
status, which SIDs have been allocated and which are available, which
SID format is used for that block, etc.). It also adds some functions to
manage the block (allocate / free / lookup).

These functions will be used in the next commits to support the
allocation of SIDs from a block in the SID Manager.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>