[feature request]: transaction_types on on /blocks/:height, and ?filter_types on /blocks/:height/transactions

[danielcolinjames]

Related to this PR for Explorer: helium/explorer#718

If we want to keep the UI for block detail pages useful (with the ability to see the breakdown of transaction types in each block, like this:

), then we would probably need 2 related API changes:

1. inside the block 'metadata' (/blocks/:height), if we could get a breakdown of the transaction types, in addition to the transaction_count field, we could populate the transaction types widget at the top with that info. For example:

{
    "data": {
        "transaction_count": 9,
        "transaction_types": {
            "poc_request_v1": 5,
            "poc_receipts_v1": 3,
            "add_gateway_v1": 1
        },
        "time": 1572472462,
        "snapshot_hash": "",
        "prev_hash": "I8LYrdL-Imq34IDwUCuMUYI2opUN9wcfJvt1hKJffzo",
        "height": 100000,
        "hash": "pqHszXWVprhh0yuIbH8usdIixQ9NiSoc1RSjP1-hu4Q"
    }
}

2. We would need the ability to filter by transaction types when fetching the transactions from a block. Right now there is just /blocks/:height/transactions, and a cursor query param. We would need a filter_types param like the /activity endpoint has.