[Modules] EventGrid: Set default values to a secure value

[lsnoddy]

Description

For compliance with NIST 800 baseline, made the following changes regarding private endpoints

• added private endpoint parameter to the parameters file for topics
• updated the parameter description in deploy.bicep
• updated the readme

Pipeline references

For module/pipeline changes, please create and attach the status badge of your successful run.

Pipeline

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• My corresponding pipelines / checks run clean and green without any errors or warnings
• 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 (readme)
• I did format my code