Extend returning statement

[Lagovas]

In this PR were removed limitation that RETURNING statements should contain only *ColName structs and allow other types, like sql literals which ignored by acra-server. According to postgresql's and mysql's docs, returning may contain same expressions that is supported by SELECT. So, were updated sql's grammar to be compatible with SelectExprs.

Due to mariadb supports returning since 10.5.0, additionally was updated image for mariadb-ssl to the fresh one 10.7.1 and removed skip condition in integration tests that didn't run tests at all for mariadb with returning cases. Because base class BaseTestCase skipped non-tls tests.

After fixing running integration tests for mariadb and updating image, I found that our index.txt contains serial from old certs, and current certs tests/ssl/mysql/mysql.crt has another one. Which were generated for engineering-demo, copied there and left here as is without updating index.txt. But our current mariadb-ssl uses old certs with serial saved in index.txt. So I copied correct certs from current cossacklabs/mariadb-ssl:10.3-1 image and placed here. Otherwise acra will deny certs in OCSP checks due to mismatch serial in index.txt and current mysql.crt

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation in the Acra documentation site or has PR on documentation repository
  with new changes
• CHANGELOG.md is updated (in case of notable or breaking changes)
• CHANGELOG_DEV.md is updated
• Benchmark results are attached (if applicable)
• Example projects and code samples are up-to-date (in case of API changes)

[Zhaars]

lgtm