feat(datasource/custom): expose tags in result so that we can use followTag

[leonismoe]

Changes

Added tags field to the schema for custom datasources.

Context

I created a repository with custom datasource, and it's expected that the dependencies always update to the latest version. but the registry always returns the latest version only, and Renovate found no updates.
As I debugged, the allSatisfyingVersions and dependency.tags in lib/workers/repository/process/lookup/index.ts are both empty, so the followTag is also unavailable for my custom datasource.
This PR fixes the issue.

Documentation (please check one with an [x])

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• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

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[renovate-release]

🎉 This PR is included in version 37.415.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.415.0

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