Add a service for checking Cody Ignore policy and decorate some UI with ignored status

[dominiccooney]

This change is integrated through Agent, but the extension side is not integrated with server-set policy yet. Instead, this change uses a testing hook which pushes an ad-hoc policy to the extension—see the test plan.

As a result, this does not cause any user visible change for Enterprise users yet. For self-serve users of the experimental file-based Cody Ignore feature, this UX may trigger until extension-side integration is complete. However we have determined that to be very few users.

Exact UX is still being designed. These labels and icons are placeholders.

Part of #1252, #1253.

Test plan

Tested manually:

1. Run the extension with CODY_JETBRAINS_FEATURES=cody.feature.internals-menu=true and open some files, say Foo.java and Bar.go
2. Status bar, Internals, Testing: Cody Ignore
3. Check "Override policy for testing", set URI regex to \.java$. OK.
4. Verify that a banner appears in Foo.java but not Bar.go.
5. Run a command like Explain Code with the right click context menu and verify that a notification appears
6. Run a command like Explain Code with a keyboard shortcut like ctrl-shift-E and verify that a notification appears
7. Verify when focus switches from Foo.java to Bar.go, a slashed Cody icon appears in the status bar when an ignored file is focused.

[odisseus]

Hmph.

[pkukielka]

LGTM