Mask the credentials when reading the stream config from streams API

The current benthos Streams API provide a good way to dynamically create and manage the streams. However, when reading an existing stream config via GET /streams/{id} endpoint, it would be better to mask all the credential fields or at least provide an option to mask them.

For example, currently when getting a stream with azure_cosmosdb as input, the response is like this:

{
  "active": true,
  "uptime": 222.438774375,
  "uptime_str": "3m42.438774667s",
  "config": {
    "input": {
      "azure_cosmosdb": {
        "account_key": "adfs",
        "args_mapping": "root = [\n  { \"Name\": \"@name\", \"Value\": \"benthos\" },\n]",
        "auto_replay_nacks": true,
        "batch_count": -1,
        "connection_string": "sdfa",
        "container": "testcontainer",
        "database": "testdb",
        "endpoint": "https://localhost:8081",
        "partition_keys_map": "root = \"blobfish\"",
        "query": "SELECT c.foo FROM testcontainer AS c WHERE c.bar = \"baz\" AND c.timestamp < @timestamp"
      }
    },
    "output": {
      "stdout": {
        "codec": "lines"
      }
    }
  }
}

Expected response would be:

{
  "active": true,
  "uptime": 222.438774375,
  "uptime_str": "3m42.438774667s",
  "config": {
    "input": {
      "azure_cosmosdb": {
        "account_key": "!!!SECRET_SCRUBBED!!!",
        "args_mapping": "root = [\n  { \"Name\": \"@name\", \"Value\": \"benthos\" },\n]",
        "auto_replay_nacks": true,
        "batch_count": -1,
        "connection_string": "!!!SECRET_SCRUBBED!!!",
        "container": "testcontainer",
        "database": "testdb",
        "endpoint": "https://localhost:8081",
        "partition_keys_map": "root = \"blobfish\"",
        "query": "SELECT c.foo FROM testcontainer AS c WHERE c.bar = \"baz\" AND c.timestamp < @timestamp"
      }
    },
    "output": {
      "stdout": {
        "codec": "lines"
      }
    }
  }
}

Both account_key and connection_string should be masked because they are marked as secret fields in the input definations.

Start at the Streams API GET /streams/{id} endpoint and trace how existing stream configurations are serialized. Inspect the input definitions for fields marked secret, then check the relevant Streams API tests or add focused coverage. Done means secret fields such as account_key and connection_string are masked while non-secret configuration values remain unchanged.