Support temporary schemas in get_schema()

[RasmusSkytte]

Intent

This PR adds the temporary argument to get_schema(), get_catalog() to allow it to get the temporary schema/catalog for the connection.

In addition, get_schema() can now infer the schema from tbl_dbi objects (by using id() internally).

Tests are also added and improved

The CI workflows for Microsoft SQL Server has also been updated over on AEF-DDF.
Before, the both the permanent and temporary databases were called "tempdb" which masked a few issues in the identification functions. The new workflows uses "master" and "tempdb" respectively, so we also fix some previously undetected issues.

The documentation of the S3 methods get_schema() and get_catalog() was adjusted to try to figure out the best way of documenting the functions. Discussion available on #69.

Approach

• The temporary parameter was added to some get_schema() methods.

• id() was used to infer schema from tbl_dbi

• Bugfixes made for table_exists() and get_tables(), specifically for MSSQL

• Better error message added for id.tbl_dbi in case table has been deleted.

Known issues

Tests for PostgreSQL are showing as failing.
This is caused by an update to the Workflows so that previously undetected errors are now being explicitly shown.
The source of the error is being fixed in #98

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR