Invoice rpc metadata support

[sputn1ck]

Change Description

This PR adds support for adding and fetching invoice metadata via lnrpc.

[gemini-code-assist[bot]]

Summary of Changes

Hello @sputn1ck, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the lnrpc system by integrating comprehensive support for invoice metadata. This feature allows users to embed arbitrary byte data directly into invoices when they are created, and subsequently retrieve this data when interacting with payment requests or querying invoice details. This provides a flexible and extensible mechanism for associating custom information with Lightning Network invoices, enabling a wider range of application-specific use cases.

Highlights

• Invoice Metadata Support: Introduced the ability to add and retrieve arbitrary metadata for both regular and hold invoices via the lnrpc interface.
• API Extensions: Extended AddInvoiceData, AddHoldInvoiceRequest, Invoice, and PayReq structures to include a Metadata field, allowing byte arrays to be attached to invoices.
• Integration Testing: Added a new integration test (testInvoiceMetadata) to thoroughly verify that metadata can be correctly set during invoice creation and subsequently fetched when decoding payment requests or looking up invoices.

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[ziggie1984]

Seems like the most straight-forward way to do it, just wanted to point out that now DB changes are needed because we pack the metadata into the invoice request which is saved as a blob in the SQL case.

[sputn1ck]

Seems like the most straight-forward way to do it, just wanted to point out that now DB changes are needed because we pack the metadata into the invoice request which is saved as a blob in the SQL case.

Do you think db changes are neccessary? The metadata is encoded in the payreq, thus is able to be fetched by the lookupinvoice call (as seen in the itest).

[ziggie1984]

Do you think db changes are neccessary? The metadata is encoded in the payreq, thus is able to be fetched by the lookupinvoice call (as seen in the itest).

Sorry for the confusion, I had a typo in my initial comment which should have been "no" instead of "now". You are correct no DB migration is needed. Maybe if we introduce Bolt 12 offers this might change but for now you pack it into the request.

[lightninglabs-deploy]

@sputn1ck, remember to re-request review from reviewers when ready