Allow HEC endpoint to be specified

[vears91]

Specify a HEC endpoint to use instead of the default
services/collector if needed.

Closes #742

Proposed changes

Add .Values.splunk.hec.endpoint and .Values.global.splunk.hec.endpoint to specify hec_endpoint in the configuration. This would allow a custom endpoint to be configured for fluent-plugin-splunk-hec. A pull request has been prepared for this feature in the plugin's repository: splunk/fluent-plugin-splunk-hec#222

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I have read the CLA
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

[hvaghani221]

This PR can only be merged after splunk_hec plugin is released