[Feat.Req] Watch mode

[pete-eiger]

Problem statement
We should add a cargo feature that enables us to run Subgraph Radio in "watch mode" where it matches all content topics (deployment hashes) in order to obtain a full view of the consensus state in the network. This way, we can fetch data from the metrics and the db (through the GraphQL API).

Expectation proposal
Watch mode should support the following:

• Shouldn't require Indexer mnemonic/pk or address to start up (afaik it shouldn't even require Graph Node endpoint?)
• Shouldn't send any messages
• Should match all incoming messages, regardless of content topic