Upgrade CI/CD (replace OpenAI->Voyage; suppress OpenAI from extended vec.ze)

[sl-at-ibm]

Pending a valid OpenAI API Key to use, this PR:

• factors in a single place the choice of the (auth-requiring) vectorize provider used in the workflow CI/CD process;
• replaces its setting to OpenAI with VoyageAI (for which we have a working API Key)
• while we're at it, suppresses (temporarily) OpenAI in its entirety from the "vectorize" portion of the integration tests (which are not run by the github automation, but still)

[sl-at-ibm]

@erichare I'm asking for your review mostly to let you know that this will unblock the CI/CD process and usher in a new flood of improvements in the form of subsequent PRs. So, all in all, very good news (and a much-needed refactor of the integration tests around vectorize).

[erichare]

Awesome, LGTM @sl-at-ibm . Very good news indeed!

[sl-at-ibm]

Well, it turned out this IT upgrade had a lot of hidden work that became apparent only after starting the overhaul, but I should be nearing completion now!
Unfortunately still some flakiness on the Astra integration test side (timeouts, metadata sync issues and such), but I'll work out a way.

[sl-at-ibm]

Force-merging because all of IT has succeeded except the downloading of coverage json from latest main commit (to compute codecov diff). "Artifact has expired".

Should be healed once this gets to main, CI runs again, and stores the new main-coverage-json for subsequent PRs.