accounting: Add desired units to node report query

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At present, the node report output query provides BTC values quoted in millisatoshis.
Multiple amount fields can be confusing, so rather than add more fields with our new units, we can deprecate the existing field in `ReportEntry` and replace it with a `oneof`. Since fiat values are subject to rounding (if we round down to BTC, we likely also want to round down our USD amount), we should paid the amount + fiat fields. Calculation in the actual accounting package should **remain in msat**, since it provides us with the best level of precision.

### Steps to completion
1. Add a `unit` enum to `NodeReportRequest` which defaults to `msat` but allows `satoshis` and `bitcoin`, round accordingly in `rpcReportResponse` and make sure the headings in the csv file are updated
1. Deprecate the `amount` and `fiat` fields and replace with a oneof:
```
entryAmount{
amount = 1;
fiat = 2;
}

oneOf{
entryAmount msatAmount =1;
entryAmount satoshiAmount = 2;
entryAmount bitcoinAmount =3;
}
```

~This change should be mindful of the intention to add other fiat currencies, as detailed in #38~

Start by locating NodeReportRequest, ReportEntry, and rpcReportResponse, then review the accounting package to confirm calculations remain in msat. Implement the requested unit selection, paired amount and fiat oneof fields, rounding, and CSV heading updates; done means msat, satoshi, and bitcoin outputs are represented correctly.

Written by the indexing model from the issue text.