feat: throw exception when user attempt to override api-version header.

[zhumin8]

As discussed offline, amend behavior so that if user code attempt to override api-version header via user header, conflict resolution in ClientContext would throw exception. This is consistent whether service itself sets api-version in proto or not (or empty).

TODO: when #2737 gets in, add a test case to ITApiVersionHeaders.java where attempts setHeaderProvider() and exception throws.

[sonarqubecloud[bot]]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud

[sonarqubecloud[bot]]

Quality Gate failed for 'java_showcase_integration_tests'

Failed conditions
50.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud