feat: support a separate storage directory for observers.sqlite

[rafaelcr]

Allows configuring a separate storage directory for observers.sqlite so state data can live separately and observer configuration is not reflected in data snapshots that might be moved to different environments.

In the config .toml file, this is now controlled by the observers_working_dir value:

[storage]
working_dir = "tmp"
observers_working_dir = "observers"

---

Additional work:

• Consolidates observer API server creation methods in a single file
• Adds unit tests for all API endpoints
• Adds validations when manipulating predicates

Fixes #274

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 57.61589% with 64 lines in your changes missing coverage. Please review.	Files with missing lines	Patch %	Lines
components/ordhook-core/src/service/http_api.rs	60.86%	45 Missing :warning:
components/ordhook-core/src/service/mod.rs	0.00%	10 Missing :warning:
components/ordhook-core/src/service/observers.rs	75.00%	3 Missing :warning:
components/ordhook-cli/src/cli/mod.rs	0.00%	2 Missing :warning:
components/ordhook-core/src/download/mod.rs	0.00%	2 Missing :warning:
components/ordhook-core/src/scan/bitcoin.rs	0.00%	1 Missing :warning:
components/ordhook-core/src/service/runloops.rs	0.00%	1 Missing :warning:

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