Introduce new API for determining the codec to use for a Postgres type

[benjie]

Description

New pgCodecs_findPgCodec hook enables searching for the PgCodec to use for a specific PgType - this enables users to add their own codecs for their own custom types (not requiring them to be added to core). This is demonstrated via PgLtreePlugin which adds the LTree GraphQL type for use with the ltree database type.

Whilst working on this, I also:

• Fixed issue with jsonwebtoken in the test script
• Improved some error messages
• Made it so plugins don't need to declare a namespace just to use cache/state
• Stripped oids from the test exports to reduce the diff on CI/etc

This PR:

• Fixes gather doesn't seem to store cache/state unless namespace exists #1947
• Fixes Ability to add codecs via plugin system #1773
• Fixes Postgres built-in type ltree is not supported #1772

Performance impact

Marginal impact on gather phase; runtime should not be impacted.

Security impact

None known.

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: d03c7d3

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

This PR includes changesets to release 6 packages

Name	Type
graphile-build-pg	Patch
graphile-build	Patch
postgraphile	Patch
graphile-utils	Patch
pgl	Patch
graphile	Patch

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