RFR: 8365086: CookieStore.getURIs() and get(URI) should return an immutable List

[Jaikiran Pai]

Can I please get a review of this change which proposes to address the issue noted in https://bugs.openjdk.org/browse/JDK-8365086?

`java.net.CookieStore` has some APIs which return a `java.util.List`. These API are specified to return an immutable `List`. The JDK ships an implementation of the `CookieStore` - the `java.net.InMemoryCookieStore`. The implementations of the `getURIs()` and `get(URI)` methods  in the `InMemoryCookieStore` currently return a `List` which is modifiable.

The changes in this PR fix those two method to return an immutable `List` to match the specification. A new regression test has been introduced to reproduce the issue and verify the fix.

I think this may not require a CSR since this fixes the implementation to match the already existing specification in `CookieStore`. I will create one if anyone suggests we should.

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Commit messages:
 - 8365086: CookieStore.getURIs() and get(URI) should return an immutable List
 - 8365086: add a test

Changes: https://git.openjdk.org/jdk/pull/26698/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26698&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365086
  Stats: 126 lines in 2 files changed: 118 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26698.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26698/head:pull/26698

PR: https://git.openjdk.org/jdk/pull/26698