Tags field for all message types

[odesenfans]

We'd like to make it possible for users to add custom tags to messages to filter their data. Today, the CCN API already provides a tags field that filters out messages by a list field in content.content.tags. PROGRAM messages also have the content.metadata field, which is a user-defined dictionary. Problems:

1. This field may or may not be defined, and if it is defined it may not be a list.
2. We don't necessarily want to add custom metadata to all the message types and keep it simple.

To make tags available to all message types, I propose to add a new tags field to the base message content model. This field would be enforced to be a list of strings, with a limit of 16 tags. We apply it to the content and not the message itself so that tags are signed.

On the nodes side, to make this feature backwards compatible with the content.content.tags support for POST messages, we would process the tags in this order:

1. Check if content.tags exists.
2. If it does not, check if content.content.tags exists and is a list of strings.

[hoh]

Must this field be signed and therefore part of the item_hash ?