Proposal for standardizing Fulcio's Certificate Extensions to align with
the discussion on [standardizing OIDC token
claims](sigstore#754) across CI/CD
systems (today GitHub Actions, in future Circle,
GitLab, Buildkite etc).

The aim here is to find new platform agnostic extensions (as the current
ones are GitHub specific) that make sense across the different CI/CD
providers that we'd like to see supported in Fulcio.

I've preemptively moved the existing oid info doc to a deprecated
version with little thought into how this transition should happen in
practice. This should probably be scoped in a lot more detail.

Signed-off-by: Philip Harrison <philip@mailharrison.com>

Co-authored-by: Phill MV <phillmv@github.com>
Co-authored-by: Fredrik Skogman <kommendorkapten@github.com>
Signed-off-by: Philip Harrison <philip@mailharrison.com>

Signed-off-by: Philip Harrison <philip@mailharrison.com>

Signed-off-by: Philip Harrison <philip@mailharrison.com>