Add IaC DB schema upgrade/downgrade scripts

[Chanel (chanel-y)]

The IaC extractor shipped no schema-migration scripts, so any mismatch between a database's dbscheme and a query pack's dbscheme was fatal (e.g. MRVA "database is not compatible with a QL library" errors). The dbscheme changed when Bicep support (82 bicep_* relations) was removed and yaml_comments/empty_location were added.

• Add upgrade step iac/ql/lib/upgrades/e360fadd.../ (old bicep -> current): deletes the 82 bicep_* relations (compatibility: backwards).
• Add downgrade step iac/downgrades/6b6bd68.../ (current -> old bicep): restores empty bicep tables, drops yaml_comments/empty_location (compatibility: partial).
• Bootstrap iac/downgrades as a pack (qlpack.yml, initial/) and add iac/ql/lib/upgrades/initial/, matching go/rust/swift layout.
• Teach misc/scripts/prepare-db-upgrade.sh about the iac language.
• Fix iac/scripts/create-extractor-pack.ps1 (was a stale QL-for-QL copy) to mirror the .sh and ship the downgrades folder.

Verified with codeql 2.26.2: upgrading an old bicep DB to the current scheme and running an IaC query succeeds; downgrading a current DB back to the old scheme succeeds.

[Mathias Vorreiter Pedersen (MathiasVP)]

I think this all makes sense. I'll just take a quick look to check if we're not being bitten by Windows vs. Unix line-endings like I fixed in #225 for PowerShell

[Mathias Vorreiter Pedersen (MathiasVP)]

Yep, the hashes seems to all match up! If you're curious about what I did to check this we can have a chat about it if you want.

This upgrade/downgrade pair conflates two separate dbscheme changes: the removal of the Bicep extensionals from the extractor (which was done before we got ownership of the extractor), and the creation of the yaml_comments extensional. That is, this is the history:

|      A        |           B               |           C
| ------------- | ------------------------- | ------------------->
^ initial       ^ Bicep removed             ^ yaml_comments added

Since the upgrade/downgrade pair in this PR takes us all the way from A to Cany database created in periodB` won't be upgradeable/downgredeable. If that's not a problem then this PR is totally fine. But if that is a problem we need to split the upgrade/downgrade into a chain of length 2.