[Response Ops][Connectors] Cases webhook connector requires hasAuth=true to pass headers to request

[ymao1]

From this PR: #183162

When HTTP headers are specified in the cases webhook connector, the above PR made a change that required the hasAuth flag to be checked in order for those headers to be included in the request. This is a breaking change as existing case webhook connectors will no longer work as expected.

Seems like it's this specific commit: 104f881

Instead of

const headersWithBasicAuth = hasAuth
    ? combineHeadersWithBasicAuthHeader({
        username: user ?? undefined,
        password: password ?? undefined,
        headers,
      })
    : {};

we can just do

const headersWithBasicAuth = combineHeadersWithBasicAuthHeader({
        username: user ?? undefined,
        password: password ?? undefined,
        headers,
      })?

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[ymao1]

We should also check out all the connectors that were touched in #183162 to ensure that the http headers are still passed as expected.

[ymao1]

cc @cnasikas

[cnasikas]

This PR #185925 fixed the bug for 8.15+. I did an audit of all connectors using combineHeadersWithBasicAuthHeader; no other connector is affected. Also, all connectors have tests to verify that additional headers are being passed properly.

[cnasikas]

PR #189583 will add the bug as a known issue to the 8.14 release notes. I am closing the issue as it is considered fixed.