Add TiDB storage backend

[ngaut]

Summary

• Add an optional TiDB storage backend and wire it through workspace, binary, config, bootstrap, serve, backup/restore, TTL, and tests.
• Make backend defaults and bootstrap options backend-owned instead of passing raw Postgres-shaped CLI arguments through shared code.
• Leverage TiDB-native capabilities: generated-column secondary indexes, snapshot-consistent reads, native TTL where stream REMOVE records are not required, and BR-backed backup/restore.
• Update docs, config samples, install scripts, and CLI lifecycle tests for generic --storage-* backend flags and TiDB guidance.

Verification

• cargo fmt --all -- --check
• git diff --check HEAD
• cargo test -j12 -p extenddb-storage-tidb
• cargo test -j12 -p extenddb --no-default-features --features tidb --all-targets
• cargo test -j12 -p extenddb --all-targets
• cargo test -j12 -p extenddb-storage-postgres
• cargo clippy -j12 -p extenddb --no-default-features --features tidb --all-targets -- -D warnings
• /tmp/extenddb-docs-venv/bin/python docs/build-docs.py

Known Existing Issue

• cargo clippy -j12 -p extenddb --all-targets -- -D warnings still fails on crates/storage-postgres/src/data/update_item.rs:168 (clippy::collapsible_else_if). This is an unrelated pre-existing Postgres data-plane lint left untouched.

[LeeroyHannigan]

Thanks for your PR. Can you please submit an RFC for this, defined in the contribution guide.

We are currently discussing how best to accompany multiple different backend storage engines which you may have seen from other PRs like this. For that reason, I'll mark this as deferred for now. However I am interested in reviewing the RFC.

[ngaut]

Thanks, addressed by opening tracking issue #141 and the RFC-only PR #142: #142. I will keep the implementation PR deferred until the RFC discussion resolves, and use the RFC thread for design review.