ADR: JMAP PublicAsset extension

Why

We should write an ADR regarding our choice of implementing the new JMAP PublicAsset extension.

where: In james-project-private repo
Explain:
- the context of why we need to implement the extension (inlined signature image broken for Gmail)
- what are the consequences (pros and cons, how would the client use the extension, security and cost issues if any, ...)
- Alternatives: e.g. CID solution - what are the drawbacks...