storage-types: reorganize sources.rs into modules

[petrosagg]

This is a pure code movement PR that splits out
src/storage-types/src/sources.rs into the following structure:

sources
├── encoding.proto
├── encoding.rs
├── envelope.proto
├── envelope.rs
├── kafka.proto
├── kafka.rs
├── load_generator.proto
├── load_generator.rs
├── postgres.proto
├── postgres.rs
├── testscript.proto
└── testscript.rs

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[shepherdlybot]

Mitigations

Completing required mitigations increases Resilience Coverage.

• (Required) Code Review 🔍 Detected
• (Required) Feature Flag
• (Required) Integration Test
• (Required) Observability
• (Required) QA Review
• Unit Test

Risk Summary:

The risk score for this pull request is high at 82, indicating a significant chance of introducing a bug based on predictors such as the average age of files, cognitive complexity within files, and the delta of executable lines. Historically, PRs with these characteristics are 124% more likely to cause a bug than the repository baseline. Additionally, there are 3 files modified that have a history of frequent bug fixes, which may contribute to the risk. The repository's observed bug trend is decreasing, but there have been recent spikes in bug fixes, while the predicted bug trend also suggests a decrease with an increase in riskier pull requests recently.

Note: The risk score is not based on semantic analysis but on historical predictors of bug occurrence in the repository. The attributes above were deemed the strongest predictors based on that history. Predictors and the score may change as the PR evolves in code, time, and review activity.

Bug Hotspots:
What's This?

File	Percentile
../catalog/migrate.rs	94
../src/pure.rs	99
../statement/ddl.rs	98

[benesch]

Nice! LGTM.

[benesch]

Force merging to get past a buf false positive: bufbuild/buf#2318 (comment).

[guswynn]

thank you!