perf(dashboard): only fetch ids and specified fragments for a relation

[kwannoel]

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What's changed and what's your intention?

Previously we fetch all fragments across all relations. Then we use it to:

1. Construct the fragment graph.
2. Support search by actor id
3. Support search by fragment id

In this PR we refactor it into:

1. Only query the required TableFragments for the specified relation_id on demand.
2. Get a hierarchy of relation_id -> fragment_id -> actor_id, and use this to support search by actor_id, fragment_id.

Part of #18176

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[kwannoel]

Provide a new proto for it to make it easier to work with on the web frontend side.

[BugenZhao]

LGTM

[tedyu]

nit: searchFragId can be used in the comparison so that we save parseInt call.