feat: Implement a simple backend for Google BigTable (#64)

First iteration of a Google BigTable backend that supports
 - Concurrent uploads and downloads
- Table management: Automatically creates the table, column families,
and GC policies.
- Overwriting objects: When calling `put` on an object that exists, the
old one will be deleted along with its metadata.
- TTL and TTI: Automatic expiry of objects after a time-to-live, as well
as automatic bumping via time-to-idle. The latter has to be implemented
as re-inserting, so the bump is debounced to once per day.
- Metadata storage: Encodes the metadata map, including custom metadata,
into a dedicated column.

Locally and in CI this is tested using the BigTable Emulator. The
emulator is also added to devservices for convenience. Unfortunately, we
had to bring back `protoc` since this is required by one of the
dependencies.

Future improvements:
- Chunking to store objects larger than 10 MB efficiently
- Multi-fetching of more than one object at a time
- (TBD) Splitting metadata into dedicated columns

Closes FS-107

Author: jan-auer

.envrc

@@ -1 +1,9 @@
+#!/bin/bash
+
+# Our policy is that the .envrc is entirely optional, and a user
+# without direnv should not really have a worse dev experience in any way.
+
+# Use with `devservices up objectstore --mode=full`
+export BIGTABLE_EMULATOR_HOST=localhost:8086
+
 source_env_if_exists .envrc.private

.github/workflows/ci.yml

@@ -24,6 +24,11 @@ jobs:
           rustup toolchain install stable --profile minimal --no-self-update
           rustup component add clippy rustfmt rust-docs --toolchain stable
 
+      - name: Install protoc
+        uses: arduino/setup-protoc@v3
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+
       - uses: swatinem/rust-cache@v2
         with:
           key: ${{ github.job }}
@@ -46,16 +51,31 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Start Bigtable Emulator
+        run: |
+          docker run -d \
+            --name bigtable-emulator \
+            -p 8086:8086 \
+            google/cloud-sdk \
+            gcloud beta emulators bigtable start --host-port=0.0.0.0:8086
+
       - name: Install Rust Toolchain
         run: |
           rustup toolchain install stable --profile minimal --no-self-update
 
+      - name: Install protoc
+        uses: arduino/setup-protoc@v3
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+
       - uses: swatinem/rust-cache@v2
         with:
           key: ${{ github.job }}
 
       - name: Run Tests
         run: cargo test --workspace --all-features
+        env:
+          BIGTABLE_EMULATOR_HOST: localhost:8086
 
   docs:
     name: Cargo Docs
@@ -72,6 +92,11 @@ jobs:
       - name: Install Rust Toolchain
         run: rustup toolchain install stable --profile minimal --component rust-docs --no-self-update
 
+      - name: Install protoc
+        uses: arduino/setup-protoc@v3
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+
       - uses: swatinem/rust-cache@v2
         with:
           key: ${{ github.job }}