docs: document directed identity for non-interactive grants

[brad-dow]

Fixes #DOC-34

Updated the following pages to document directed identity:

• client-keys.mdx - key registry, client request body, grant requests flow, and sequence diagram intro
• grants.mdx - clarify that directed identity clients provide their public key directly and don't require a key registry
• auth.mdx - included directed identity as an option for incoming payment and quote grant requests

Also, bumps the open-payments-specifications submodule to pick up the latest spec changes, including directed identity

Checklist

• PR title is prefixed with docs:
• PR title or description includes a fixes #
• You've run pnpm format and pnpm lint
• You've reviewed Vale errors and made changes where appropriate

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[melissahenderson]

Looks good overall. Do you think we should add directed identity to the glossary?

[melissahenderson]

Who is "their"? Clients or requests?

[brad-dow]

Good catch...I'll add "the client's identity".

[melissahenderson]

Was this a Prettier addition?

[brad-dow]

I installed claude code locally for some experimentation, and I'm pretty sure it was an artifact from that. I pulled that back out, so it shouldn't show up anywhere else.

[brad-dow]

Looks good overall. Do you think we should add directed identity to the glossary?

I waffled a bit on whether to include it or not. I feel like the rest of the glossary terms are foundational to Open Payments. Things like ASE and infrastructure pieces like auth server, etc. Directed identity feels like a layer deeper than those. However, because it's a "thing with a name" now across multiple pages it's worth including? I could go either way.