[SECRT-1896: Fix crazy `GET /api/graphs` latency (P95 =
107s)](https://linear.app/autogpt/issue/SECRT-1896)
These changes should decrease latency of this endpoint by ~~60-65%~~ a
lot.
### Changes 🏗️
- Make `Graph.credentials_input_schema` cheaper by avoiding constructing
a new `BlockSchema` subclass
- Strip down `GraphMeta` - drop all computed fields
- Replace with either `GraphModel` or `GraphModelWithoutNodes` wherever
those computed fields are used
- Simplify usage in `list_graphs_paginated` and
`fetch_graph_from_store_slug`
- Refactor and clarify relationships between the different graph models
- Split `BaseGraph` into `GraphBaseMeta` + `BaseGraph`
- Strip down `Graph` - move `credentials_input_schema` and
`aggregate_credentials_inputs` to `GraphModel`
- Refactor to eliminate double `aggregate_credentials_inputs()` call in
`credentials_input_schema` call tree
- Add `GraphModelWithoutNodes` (similar to current `GraphMeta`)
### Checklist 📋
#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `GET /api/graphs` works as it should
- [x] Running a graph succeeds
- [x] Adding a sub-agent in the Builder works as it should