Better validation for API requests.

[oliverchang]

We can validate:

1. The provided version is valid for the given ecosystem. We can simply do this by attempting to parse the provided version, if an ecosystem is specified.
2. The provided package and version number exists in the ecosystem (this will need https://deps.dev support).

[andrewpollock]

Based on my read of b/291986718, also when an invalid/unsupported ecosystem is provided

[github-actions]

This issue has not had any activity for 60 days and will be automatically closed in two weeks

[github-actions]

Automatically closing stale issue