feat(api): Implement validation endpoints

[iambriccardo]

This PR introduces validation endpoints for destinations, pipelines, and their combined configuration, with the goal of making it easier for users to verify that all required preconditions are satisfied before a pipeline is created or executed.

The validation mechanism is implemented through three new endpoints, each responsible for validating a pipeline, a destination, or both together. These endpoints are intended to be called prior to resource creation, allowing clients to determine upfront whether a configuration is viable and whether all prerequisites are met. The system is structured to support incremental validation steps, making it straightforward to extend with additional checks over time.

The design follows the principle that the replicator itself should not perform validation, and should instead fail fast when encountering invalid configurations. This keeps the replicator code simpler and avoids negatively impacting startup performance, which would otherwise be affected by executing potentially expensive validation logic during initialization

[iambriccardo]

Moved here.

[coveralls]

Pull Request Test Coverage Report for Build 20738127653

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 305 of 455 (67.03%) changed or added relevant lines in 17 files are covered.
• 434 unchanged lines in 24 files lost coverage.
• Overall coverage decreased (-0.2%) to 81.393%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
etl-api/src/routes/destinations_pipelines.rs	10	11	90.91%
etl-api/src/validation/validators.rs	68	69	98.55%
etl-destinations/src/bigquery/client.rs	10	11	90.91%
etl-replicator/src/core.rs	0	2	0.0%
etl-config/src/environment.rs	0	5	0.0%
etl-examples/src/main.rs	0	7	0.0%
etl-api/src/main.rs	0	8	0.0%
etl-replicator/src/main.rs	0	8	0.0%
etl-api/src/validation/mod.rs	55	65	84.62%
etl-destinations/src/bigquery/core.rs	0	20	0.0%

Files with Coverage Reduction	New Missed Lines	%
etl-replicator/src/core.rs	2	0.0%
etl-api/src/config.rs	3	26.09%
etl/src/workers/pool.rs	3	92.96%
etl-postgres/src/replication/worker.rs	5	61.54%
etl-config/src/load.rs	6	82.97%
etl-postgres/src/replication/state.rs	7	96.62%
etl/src/test_utils/notify.rs	8	90.78%
etl/src/test_utils/test_destination_wrapper.rs	8	84.71%
etl-api/src/startup.rs	9	58.58%
etl-destinations/src/iceberg/core.rs	9	95.56%

Totals
Change from base Build 20601971115:	-0.2%
Covered Lines:	17126
Relevant Lines:	21041

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💛 - Coveralls