Introduce concept of SQL-able to pg-sql2

[benjie]

Description

Writing sql`${$user.getClassStep().alias}.is_admin is true` is a chore; this change makes it so that sql can recognize embedded values that can be turned natively into SQL, for example pgSelect and pgSelectSingle can now be used like: sql`${$user}.is_admin is true` which is much simpler and easier to read.

Note: this is only useful for aliases, you cannot use this for placeholders.

Performance impact

Negligible, planning only.

Security impact

I guess you could argue that it's slightly more magic, and thus slightly riskier that the coder might do the wrong thing? I can't see how this would turn into a vulnerability that would get past testing though.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

🦋 Changeset detected

Latest commit: 7819fde

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 15 packages

Name	Type
graphile-build-pg	Patch
postgraphile	Patch
@dataplan/pg	Patch
pg-sql2	Patch
graphile-utils	Patch
pgl	Patch
graphile	Patch
@localrepo/grafast-bench	Patch
@dataplan/json	Patch
@grafserv/persisted	Patch
grafserv	Patch
ruru-components	Patch
@localrepo/grafast-website	Patch
graphile-build	Patch
graphile-export	Patch

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