Expose package ecosystem specific messages for deprecation

As a follow-up to https://github.com/google/deps.dev/issues/92 it would be useful, if possible, to expose the developer-documented reason for a deprecation of a package.

i.e. https://www.npmjs.com/package/@material-ui/core notes:

> Material UI v4 doesn't receive active development since September 2021. See the guide https://mui.com/material-ui/migration/migration-v4/ to upgrade to v5.

Compare to https://www.npmjs.com/package/@jamietanna/renovate-one-off (which does not note any deprecation reason):

> Package no longer supported. Contact Support at https://www.npmjs.com/support for more info.

And a Go module (https://pkg.go.dev/github.com/deepmap/oapi-codegen):

> Deprecated: use github.com/deepmap/oapi-codegen/v2 as major version v1 is no longer maintained. The upgrade path is smooth (see https://github.com/deepmap/oapi-codegen/releases/tag/v2.0.0 for more details) and only introduces a reduced set of dependencies, as well as future releases introducing a large number of enhancements and bug fixes

Adding a `deprecationReason` or `deprecationMessage` field to the response would be very useful