fix: preemptively send basic auth

[yaauie]

When Elasticsearch is configured to allow anonymous auth, it does not include a challenge header in response to requests, so we cannot rely on the apache http client's auth handling (which is overkill anyway, since an individual Elasticsearch client is not connected to multiple realms).

Instead, we build the http basic auth header ourselves, and register an interceptor to ensure it is added to all requests, similar to how we handle api key auth headers.

[elasticmachine]

:broken_heart: Build Failed

• Buildkite Build
• Commit: 9eeea439163c4381f0b2a1205c8bfd13f0f4b2c3

Failed CI Steps

• :hammer: CI setup and unit tests run :docker:
• :hammer: CI setup and integration tests with ES 8.x-SNAPSHOT run on :docker:

History

• :broken_heart: Build #275 failed 8ff49b10b4ef1cef2ff18c2841bffd1eb9778d75

[yaauie]

Buildkite failed because it is trying to build against Elasticsearch 8.13's source, but the 8.13 branch has not yet been cut. When building against Elasticsearch 8.11's source (as done in our gradle tasks for building and releasing the artifact), tests succeed.