LIP-003: Message format V3

[Ronsor]

This proposes a new optionally backward-compatible message format (LNMSGV3/flag 131072)

In a backwards compatible form, LNMSGV3 is encapsulated in a LNMSGV2 (current-format message), with the flag 131072 (LNMSGV3-RICH). Then a new field is introduced in the payload: bodyhtml. This field contains an HTML version of the message. The normal body field contains a plaintext version. This will however, double the size of messages and increase fees.

Without backwards compatibility, LNMSGV3 is the same format as LNMSGV2, with a LNMSGV3 payload type and bodyhtml in the payload instead of body. Flag 131072 must NOT be specified.

Misc notes:

• HTML messages can contain the following types of formatting at minimum: headers (h1-h6), bold (b/strong), italic (i, em), underline, strikethrough, images with data URLs (types: image/png, image/jpeg, image/gif).