feat(codegen): Allow plugins to access environment variables

[kyleconroy]

Allow plugins, both process-based and WASM-based, to read environment variables from the host, configured via a per-variable allowlist. Plugins can not declare the environment variables they have access to outside of the sqlc.yaml configuration file.

version: '2'
plugins:
- name: py
  env:
  - PATH
  wasm:
    url: https://github.com/sqlc-dev/sqlc-gen-python/releases/download/v0.16.0-alpha/sqlc-gen-python.wasm
    sha256: 428476c7408fd4c032da4ec74e8a7344f4fa75e0f98a5a3302f238283b9b95f2

Closes #2661

[andrewmbenton]

So the idea here is that the user can decide which variables go in the allow list, via sqlc.conf?

[kyleconroy]

Exactly. This is very similar to how wasmtime allows environment variables to be accessed.

version: '2'
plugins:
- name: py
  env:
  - PATH
  wasm:
    url: https://github.com/sqlc-dev/sqlc-gen-python/releases/download/v0.16.0-alpha/sqlc-gen-python.wasm
    sha256: 428476c7408fd4c032da4ec74e8a7344f4fa75e0f98a5a3302f238283b9b95f2
sql:
- schema: "schema.sql"
  queries: "query.sql"
  engine: postgresql
  codegen:
  - out: src/authors
    plugin: py
    options:
      package: authors
      emit_sync_querier: true
      emit_async_querier: true
      query_parameter_limit: 5

[andrewmbenton]

There is the SQLC_VERSION var which is added (not from the actual shell environment though). That should be documented somewhere, but probably not here.