[KB] add KB support to fleet package spec and implementation

[pgayvallet]

We're going to use fleet packages as the installation/management mechanism for our KB "sources".

However atm, the fleet package specification does not provide everything we would need to achieve this, so we will have to update the spec, and then adapt the implementation accordingly.

The discussion on the package-spec changes: elastic/package-spec#693

A POC done by @spong to add a KB integration: elastic/integrations#9007

We will likely need to perform changes in elastic-package and package-registry too, at least for local testing

[elasticmachine]

Pinging @elastic/appex-ai-infra (Team:AI Infra)

[pgayvallet]

package-spec PR: elastic/package-spec#807
fleet PR: #194209
knowledge-base PR: #194379

[pgayvallet]

Closing - we won't be doing this for the MVP