Data-connector and Database-proxy reconciliations

[Sylvestre67]

Resolves https://github.com/observablehq/projects/issues/421 https://github.com/observablehq/projects/issues/423 https://github.com/observablehq/projects/issues/422

• [x] Add SNOWFLAKE_TEST_CREDENTIALS as secret on GitHub Actions for the test to pass.

See related pr on data-connector: https://github.com/observablehq/observablehq/pull/10168

Includes https://github.com/observablehq/database-proxy/pull/68.

• Setup test GH actions workflows.
• Migrate general pooling from data-connector to database-proxy.
• Migrate MySQL tests from the data-connector, and refactor to use mocha
• Cleaned up SQL Server tests.
• Migrate Postgres client method from the data-connector
• Migrate Postgres tests from the data-connector, and refactor to use mocha
• Migrate Snowflake client method from the data-connector
• Migrate Snowflake tests from the data-connector, and refactor to use mocha
• Add linter and fix format inconsistencies