chore: Update buf config to v2 (#20558)

* chore: Move third-party proto files into their own module

To avoid cyclical dependencies between lib/vector-core/proto and proto/

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

* Fix vector-core build.rs

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

* Fix prometheus-parser build.rs too

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

* loki too

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

* chore: Update buf config to v2

Via `buf config migrate`

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

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Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>