Add proxy related configurations for splunk connector config

[wangshu3000]

Problem:

Current Splunk connector doesn't have proxy related settings, that means the connector cannot connect to HEC endpoint via a proxy configured at connector level.

For some cases, to connect to splunk cloud HEC endpoint will need to go through a dedicated outbound proxy. It could be a different proxy than other connectors.

It's not supported to configure a connector level proxy via the rest configuration api.
Configure the proxy via JVM OPTS will impact all other connectors running on the same cluster.

So provide proxy related settings via rest api or connector properties would be necessary.

Many other http based connectors also have this feature.
For example: http connector, S3 connector, SQS connector etc.

Solution:

1. Add proxy settings to SplunkSinkConnecorConfig.java so that we can configure the connector's proxy via api or connect properties file. The new configurations will be together with the existing httpKeepAlive config.
2. Add proxy settings to RequestConfig in HttpClientBuilder.java file if the proxy settings were configured.
3. Other dependencies files and Test files etc.
4. Update README.md file
5. Update latest coverage report.

I would like to work on this and contribute a PR once i finish it.

[OM-HASE]

@wangshu3000
Could you please assign this issue to me? I would be very grateful for the opportunity to contribute.
Thank you!

[wangshu3000]

@OM-HASE Thanks for your interests. Looks like this is a feature we all desire. but i'm sorry to say that I have finished the changes and reviewing it, once it's all done i'll push to this repo.