feat: show pending balance on channels api

[bonomat]

With this we see if there are any pending/stuck HTLCs on an channels.

E.g. response looks like this for a channel with a pending HTLC

{
  "channel_id": "cecff04ded3279da7638d82b5abeac2e4a878ed775f60525f9e21e910c6ca7e2",
  "counterparty": "02ab77e8a40feef78116c4f6afbe5586e7e5954cf9d9425514fe757955c2c8537a",
  "funding_txo": "e2a76c0c911ee2f92505f675d78e874a2eacbe5a2bd83876da7932ed4df0cfce:0",
  "original_funding_txo": null,
  "channel_type": "StaticRemoteKey: required, AnchorsNonzeroFeeHtlcTx: not supported, AnchorsZeroFeeHtlcTx: not supported, Taproot: not supported, SCIDPrivacy: not supported, ZeroConf: not supported, unknown flags: none",
  "channel_value_satoshis": 200000,
  "unspendable_punishment_reserve": 2000,
  "user_channel_id": 1.2457119903510981e+38,
  "feerate_sat_per_1000_weight": 253,
  "balance_msat": 98879000,
  "outbound_capacity_msat": 96879000,
  "next_outbound_htlc_limit_msat": 96253000,
  "inbound_capacity_msat": 88000000,
  "confirmations_required": 0,
  "force_close_spend_delay": 144,
  "is_outbound": true,
  "is_channel_ready": true,
  "is_usable": true,
  "is_public": false,
  "inbound_htlc_minimum_msat": 1,
  "inbound_htlc_maximum_msat": 196000000,
  "config": {
    "forwarding_fee_proportional_millionths": 50,
    "forwarding_fee_base_msat": 0,
    "cltv_expiry_delta": 42,
    "max_dust_htlc_exposure_msat": {
      "FeeRateMultiplier": 5000
    },
    "force_close_avoidance_max_fee_satoshis": 1000
  },
  "scid": 205608674459648,
  "user_email": "",
  "channel_balances": [
    {
      "MaybeTimeoutClaimableHTLC": {
        "amount_satoshis": 11121,
        "claimable_height": 273,
        "payment_hash": "9cc07d10a6ea59fbafb5a4762eb1f6c1a3435bdcf4504d017a93b4919f3e2dee"
      }
    },
    {
      "NotYetClosedClaimableOnChannelClose": {
        "amount_satoshis": 98653
      }
    }
  ]
}

[holzeis]

LGTM 👍