feat: add _meta provider metadata to AgentCustomization across clients

[DonJayamanne]

No description provided.

[connor4312]

@DonJayamanne not opposed to this in principal but I'm curious why we need this already -- is there a non-standard field we want to talk about?

[DonJayamanne]

y -- is there a non-standard field we want to talk about?

Had a chat with @aeschli and almost all of the fields are non standard or we have different interpretations
E.g. event the prompt isn't a standard field.
We treat model as an array, others & spec treats it as a single value
Tools have different meanings as well.

TLDR: Based on my discussions with martin, there aren't any, hence his suggestion was to not add any property to this spec

Phase 1

• So i opted to go with the simplest to unblock from the perf issue of VS Code reading the file everytime
  I want to get this , as we have a perf issue with VS Code (client) reading the files everytime (& lots of AHP messages sent around)

Phase 2

• Figure out what are the standard properties, we must have some, I'd leave phase 2 as a debt issue to remove this content property