mysql: preserve cleartext password auth setting

[huynhtrungcip]

Changes

• Preserve the existing Go CDK default AllowCleartextPasswords behavior for generic mysql:// URLs when the option is omitted.
• Respect an explicit allowCleartextPasswords=false URL setting instead of overwriting it after DSN parsing.
• Continue allowing cleartext password authentication in awsmysql, where the connector configures TLS before connecting.
• Add coverage for default, explicit false, and explicit true allowCleartextPasswords URL settings.

Tests

• go test ./mysql -run TestConfigFromURL -count=1
• go test ./mysql ./mysql/awsmysql ./mysql/azuremysql -count=1
• go test ./mysql/... -count=1
• go test ./... -count=1

[vangent]

Can you explain why you think this change is necessary?
Isn't it a breaking change for any non-AWS mysql user that's got a cleartext password but doesn't have it explicitly in the URL?

[huynhtrungcip]

Good point. The previous version would have changed the default behavior for generic mysql:// URLs, so I adjusted the PR to avoid that compatibility break.

The updated patch keeps the existing Go CDK default when allowCleartextPasswords is omitted, but preserves an explicit URL setting such as allowCleartextPasswords=false instead of overwriting it after DSN parsing. awsmysql still sets AllowCleartextPasswords explicitly because that connector configures TLS before connecting.

Rechecked locally:

• go test ./mysql -run TestConfigFromURL -count=1
• go test ./mysql ./mysql/awsmysql ./mysql/azuremysql -count=1
• go test ./mysql/... -count=1
• go test ./... -count=1