add subscription based cache refresh

[ghost]

add subscription based cache refresh.

The current design is optimisic and friendly, it assumes that nobody will spam cache_nft for invalid CIDs.

subscribe specifies the port and protocol used for a callback, the ip address is implied.

unsubscribe removed the ip address from the callback.

has_template_cached returns true if a specific nft template is already cached, based on a CID.
this will internally call get_nft_template_list, which lists all known templates.

cache_template instructs the node to cache a specific nft template, based on a CID.
this will internally call get_nft_template_owner, which caches all template related information.

• add subscribe {"action": "subscribe", "protocol":"^http|https$", "port":"^[0-9'+$"}
• add unsubscribe {"action": "unsubscribe"}
• add has_template_cached {"action": "has_template_cached", "cid", "<cid>"}
• add cache_template {"action": "cache_template", "cid", "<cid>"}

[Airtune]

Maybe it would make more sense for the node to simply cache all chains for a template. In most cases this will be called after announcing the template on the Banano ledger and might not even have any sends yet.
I suggest:
cache_assets {"action": "cache_assets", "ipfs_cid": "<nft-template-ipfs-cid>"}
has_assets_cached{"action": "has_assets_cached", "ipfs_cid": "<nft-template-ipfs-cid>"}