Mysql storage

[twocs]

https://github.com/plutov/formulosity/issues/29

For a hacktoberfest contribution, I have implemented mysql storage. It was a bit tricky to work past mysql limitations, but now all the tests are passing. This pull request introduces support for MySQL alongside existing support for PostgreSQL and SQLite. The changes span configuration updates, new database migrations, and modifications to the service initialization logic.

Database Support Expansion:

• README.md: Updated DATABASE_TYPE to include mysql and revised the description for DATABASE_URL to include MySQL connection strings.
• api/go.mod: Added github.com/go-sql-driver/mysql and filippo.io/edwards25519 as indirect dependencies.

MySQL Migrations:

• api/migrations/mysql/000001_schema.up.sql: Created surveys table with columns for survey metadata.
• api/migrations/mysql/000002_schema.up.sql: Created surveys_sessions table with columns for session data and a foreign key reference to surveys.
• api/migrations/mysql/000003_schema.up.sql: Created surveys_questions table with columns for question data and a foreign key reference to surveys.
• api/migrations/mysql/000004_schema.up.sql: Added a unique index on surveys_questions for survey_id and question_id.
• api/migrations/mysql/000005_schema.up.sql: Created surveys_answers table with columns for answer data and foreign key references to surveys_sessions and surveys_questions.
• api/migrations/mysql/000006_schema.up.sql: Added a unique index on surveys_answers for session_id and question_id.

Service Initialization:

• api/pkg/services/services.go: Added support for initializing a MySQL storage backend based on the DATABASE_TYPE environment variable.

Docker Configuration:

• compose.yaml: Updated the api service to depend on MySQL and added configuration for a MySQL service with health checks and volume mappings. [1] [2]

Some known issues:

1. The docker-compose file is getting a bit messy now. End users are expected to fill in their data, and comment out various lines corresponding to databases they don't use. It might be nice to have multiple sample docker-compose files, one for each database service type.
2. Mysql does not appear to support multiStatement migrations (multiple DDL in a single file). The workaround I have seen (and used here) is to have a single statement per file, so there are six schema.up.sql files rather than a single one.

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[plutov]

Thanks for working on this @twocs However I already replied in he ticket that we don't need it.

[twocs]

I guess we don't need formality either, because it doesn't support our database.