[feature]: allow saving private notes for payments

[AndySchroder]

Is your feature request related to a problem? Please describe.

1. Payee doesn't always put a good memo or any memo at all on invoices. Payer should have the ability to add their own notes for their own records to all outbound payments, whether success or failure.

2. Payee wants to store more detailed information or private information that can't be placed in the memo that is shared with the payer.

Describe the solution you'd like

1. Add an RPC command that allows me to attach a private note for any outbound payment, while sending payment and after payment.

2. Allow adding a private note when creating an invoice using AddInvoice and AddHoldInvoice .

3. Allow querying a list of inbound and outbound payments based on their private notes.

Describe alternatives you've considered

Separate databases. These are a nuisance because they require another database, daemon, network socket, and authentication setup for all users of the LND node to also manage private notes.

Additional context

Zeus has an option to store a private note, but the data does not persist on the LND node, it is only on the phone the note is written on (ZeusLN/zeus#2410) . This is just one example.

[saubyk]

2. Allow adding a private note when creating an invoice using AddInvoice and AddHoldInvoice

How will this be different from the memo that you can add along with the invoice? Trying to understand the implication of 'private'?

[AndySchroder]

2. Allow adding a private note when creating an invoice using AddInvoice and AddHoldInvoice

How will this be different from the memo that you can add along with the invoice? Trying to understand the implication of 'private'?

Well, it won't really be part of the actual BOLT 11 invoice that is shared, just part of the local database. Anyone with an invoice macaroon on the LND node will be able to read it, but it otherwise won't be shared since there is no additional BOLT11 invoice fields to even put it in.

This field could not be private too if the payee wants to share it some other way than via BOLT 11. For example, it could also be used to store data that is too long to put in the invoice itself. We have a description_hash for longer amounts of data that can't fit in BOLT 11 invoices, but there isn't anywhere in LND to actually store the longer data that is hashed.