ci: db: oracle workflow

[detule]

Hi -

This establishes an oracle pipeline.

• Unable to run the DBItest suite at this time - see Oracle support DBItest#348
  • Despite this, I think it is useful - provides an environment to test any custom oracle logic, and over time we can also write tests that test more generic functionality. I am hopeful we can also figure out how to run DBItest against this backend.
• Currently workflow fails - needs oracle: tables: do not escape when using exact query #712 to pass.

────────────────────────────────────────────────────────────────────────────────
Failure ('test-driver-oracle.R:24:5'): Oracle
dbExistsTable(con, "mtcars_test") is not TRUE

`actual`:   FALSE
`expected`: TRUE 
Backtrace:
    ▆
 1. └─base::eval(...)
 2.   └─base::eval(...)
 3.     └─testthat::expect_true(dbExistsTable(con, "mtcars_test")) at test-driver-oracle.R:24:5

Error ('test-driver-oracle.R:26:7'): Oracle
<odbc::odbc_error/C++Error/error/condition>

[hadley]

Feel free to sequence the merges however you want; given that the builds are already failing, I think it's fine to merge this first.

I'll try and find some time today to do a patch for the DBI issues.