Add secrets support

[aaronsteers]

This has two phases:

Phase I. Add get_secret() support.

This allows retrieving from env vars or secret managers.

Phase 2. Auto-detect and inject

In this phase, presumably we'd be able to auto-inject secrets if they match a specific naming convention that aligns with the source name and setting name.

Tech Spec here:

https://docs.google.com/document/d/1rqoRuGRDvSYX8TmtxvpJv-XJgzfN46fLYtuMymz-QE8/edit#heading=h.c4ztxwwo0ign

[aaronsteers]

First phase (get_secret()) has shipped:

• AirbyteLib: Add basic secrets management airbyte#34822

Second phase (auto-detect and inject) has a proposal in the Tech Spec here:

• https://docs.google.com/document/d/1rqoRuGRDvSYX8TmtxvpJv-XJgzfN46fLYtuMymz-QE8/edit#heading=h.c4ztxwwo0ign

[aaronsteers]

Closing phase 1 of this, and will open this as a separate issue:

Phase 2. Auto-detect and inject

In this phase, presumably we'd be able to auto-inject secrets if they match a specific naming convention that aligns with the source name and setting name.

Tech Spec here:

https://docs.google.com/document/d/1rqoRuGRDvSYX8TmtxvpJv-XJgzfN46fLYtuMymz-QE8/edit#heading=h.c4ztxwwo0ign