Do not persist all fabrics as a single TLV

[ivmarkov]

Currently, all fabrics are persisted in the KvBlobStore by first converting them to one large TLV list and then saving the bytes' representation of this list under a single key - fabrics.

The problems with that:

• (High Prio) - some KvBlobStore implementations (notably, the sequential_map one in downstream rs-matter-embassy) have a hard limit on the size of the value being persisted. For sequential_map it is a bit less than 4K which is the NOR flash page size (or NOR flash erase size). Therefore, the current approach cannot even support > 2 fabrics, as they overflow the 4K NOR flash page;
• (Mid Prio) - a relatively large KV Buffer is necessary for the persistence to work. Roughly 2x the size of all fabrics;
• (Low Prio) a change to fabric X is requiring the persistence of ALL fabrics currently.

Possible solutions:

• One option would be to retire the sequential_map storage in rs-matter-embassy in favor of another one. Say - embassy kv;
• Another option would be to change the persistence of the fabrics in such a way that they are persisted under separate keys.