Internal and external IPFS gateway URL support

[wesbiggs]

IpfsService was recently changed (#821 ) to perform a HEAD request on the IPFS gateway's public URL. Because of this, the (external) ipfsGatewayUrl needs to be reachable by the content-publishing service, which theoretically might not always be the case depending on routing setup.

Similarly, content-watcher needs some attention as well. Currently it uses the IPFS node API to fetch content by CID; however, that requires having access to a private node--technically, running content-watcher should only require a public gateway. In which case, we would probably change content-watcher to also use ipfsGatewayUrl, which would mean it would need to be resolvable by content-watcher as well, so the same question applies.

TL;DR:

• content-publisher needs BOTH (or the same) a publicly-resolvable IPFS Gateway URL AND an internally-resolvable IPFS Gateway URL
• content-watcher needs ONLY an internally-resolvable IPFS Gateway URL

Suggested approach:

• Add optional IPFS config parameter internalGatewayUrl, templated same as gatewayUrl. If configured, this will be used for internal service access to the IPFS HTTP gateway.
• Change content-watcher to use gateway interface rather than RPC API following this approach.
• Change IpfsService to use internal gateway if configured for existsInLocalGateway()