RFC: Funding executable type in the product manifest

[BigTava]

Summary

• Adds Funding as a fourth executable type in the product manifest RFC, published under funding.<product_id>.<tld> like the other executables.
• A Funding manifest declares modes: the rails the surface supports (CARD, BANK, CRYPTO).
• Hosts ignore unrecognised modes; a manifest with no recognised mode is never offered for funding.
• The runtime contract behind the Funding surface is deferred to a dedicated Funding Modality RFC.
• Deduplicates the RFC: conformance fixtures defer to the corner cases, publisher validation carries one strictness note, and the config types are defined by reference to the manifest types.

Example

Root manifest, text record manifest on example.dot:

{
  "$v": 1,
  "displayName": "Example",
  "description": "Buy and sell cash with euros.",
  "icon": { "cid": "bafk...", "format": "png" }
}

Funding executable manifest, text record executable on funding.example.dot:

{
  "$v": 1,
  "appVersion": [1, 2, 0],
  "kind": "funding",
  "modes": ["CARD", "BANK"]
}

[filvecchiato]

the EUR denomination needs to be abstracted in the docs