feat: dm_from_con(learn_keys = TRUE) works for MariaDB

[krlmlr]

for relational.fit .

Closes #366. Closes #1124.

[krlmlr]

@maelle: Please feel free to add to this branch.

[maelle]

Noting that I do not get any failure locally but I do remember the text about MariaDB needs to be updated anyway.

[maelle]

Oh, actually I need to try lazy = FALSE.

[maelle]

@krlmlr in the dm.Rmd vignette, do you want to

• remove mentions of how to add primary keys
• change the example DB to be able to demonstrate how to add primary keys?

[maelle]

@krlmlr should the tests that have skip_if_src_not(c("mssql", "postgres")) have been updated or do they exclude MariaDB for other reasons?

It might be nice to replace skip_if_src_not(c("mssql", "postgres")) with something more self-explanatory?