chore: declare GreptimeDB Enterprise License for enterprise-gated sources (#8364)

* chore: declare GreptimeDB Enterprise License for enterprise-gated sources

The `enterprise`-feature-gated sources (triggers, mito2 extension) were
excluded from the Apache-2.0 header check but carried no license of their
own. Declare a separate GreptimeDB Enterprise License and enforce it.

- Add LICENSE-ENTERPRISE (open-core split; core stays Apache-2.0).
- Add an Enterprise License header to each enterprise source file.
- Add licenserc-enterprise.toml and a second hawkeye step in CI to enforce
  the Enterprise header on exactly those files.
- Cross-reference the two complementary file lists; document the layout in
  licenses/README.md and the README License section.

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

* chore: reference per-customer Enterprise Agreement instead of a terms URL

There is no public enterprise-terms page; each customer signs an individually
negotiated agreement. Point the license at a "separate written commercial
agreement with GrepTime Inc." and direct readers to the existing contact page.

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

---------

Signed-off-by: Dennis Zhuang <killme2008@gmail.com>

.github/workflows/develop.yml

@@ -46,6 +46,10 @@ jobs:
         with:
           persist-credentials: false
       - uses: korandoru/hawkeye@v5
+      - name: Check enterprise license header
+        uses: korandoru/hawkeye@v5
+        with:
+          config: licenserc-enterprise.toml
 
   check:
     if: ${{ github.repository == 'GreptimeTeam/greptimedb' }}