Honour Extension schema on create and update

[chlunde]

Description of your changes

spec.forProvider.schema on Extension was accepted by the CRD but never read. Every extension landed in the connection's default schema, and non-relocatable extensions (postgis) cannot be moved afterwards.

• Create appends SCHEMA <name> to CREATE EXTENSION.
• Observe reads pg_namespace.nspname next to extversion, compares it verbatim with the spec, and late-initialises schema like version.
• Update moves relocatable extensions with ALTER EXTENSION … SET SCHEMA. For non-relocatable ones the engine's does not support SET SCHEMA surfaces in Synced. Update re-observes first, so a version-only mismatch still issues no statement.
• Both trees (cluster and namespaced), CRD doc comment regenerated.

Semantics worth a reviewer's attention:

• The schema must exist. A missing schema fails Create with postgres' schema "x" does not exist.
• Comparison is exact string equality on the raw identifier. Quoting on write (QuoteIdentifier) and nspname on read round-trip mixed case, spaces and hyphens.
• Existing resources without schema late-initialise to their current schema (usually public); nothing moves.
• Existing resources that already had schema set but ignored will now try to move the extension. Relocatable: it moves. Non-relocatable: Synced=False with the engine error until the spec matches reality. Before upgrading, compare specs against SELECT extname, extnamespace::regnamespace FROM pg_extension and fix or drop schema where they disagree.

Fixes #440

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review. (Locally as GOTOOLCHAIN=go1.26.1 make lint, make test, make check-diff; the pinned golangci-lint build panics under Go 1.27.)

How has this code been tested

• Unit (reconciler_test.go, both trees): Observe schema mismatch, Create statement with version and quoted schema, Update no-op when the schema matches and ALTER EXTENSION … SET SCHEMA when it differs.
• E2E core flow: the ltree example now installs into my-schema; check_extension_test asserts hstore=public ltree=my-schema from pg_extension. Stock postgres:18 image, no new dependencies.
• Temporary e2e tests (not part of this PR; on my fork, branch worktree-custom-db-scripts, cluster/local/scripts/postgres-issue-440-extension-schema/) against imresamu/postgis:18-3.6 (postgres 18, postgis 3.6.1), both cluster and namespaced passes:
  1. postgis → gis, hstore → "Mixed Case", schema-less pg_trgm → public and late-init schema: public; ltree → missing schema stays not Ready with schema "nope" does not exist.
  2. Forced re-reconcile keeps Ready with an unchanged transition time.
  3. Out-of-band ALTER EXTENSION hstore SET SCHEMA public is reverted by Update.
  4. Patching postgis to schema: public gives Synced=False with does not support SET SCHEMA; patching back recovers.
  5. Deleting all leaves no extension behind; schemas survive.

DB_TYPES=postgresql make e2e green on both passes (cluster 118 s, namespaced 111 s), temporary tests i440: PASS on both.