x-pack/filebeat/input/entityanalytic/provider/okta: fix use of configured http client

[chemamartinez]

Proposed commit message

x-pack/filebeat/input/entityanalytic/provider/okta: fix use of configured http client

The OAuth2 authentication flow in the Okta entity analytics provider
was ignoring the user-configured HTTP client. Instead, it was using
Go's default HTTP client for all token-related requests
(initial token exchange, token refresh, and API calls).

This meant that any custom TLS/SSL or proxy settings configured by
the user were silently discarded, causing connection failures in
environments that rely on custom certificates or proxies.

This fix ensures the configured HTTP client is propagated through
all OAuth2 token operations, so that outgoing requests correctly
use the user's transport configuration.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

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