fix(synthetics): alter []SyntheticsCustomHeaderInput to allow empty custom headers in the request

[vagrawal-newrelic]

Added the capability to delete all custom headers for simple/browser synthetic monitors. changes as of this PR:

• updated the type of []SyntheticsCustomHeaderInput type to pointer.
• relevant changes to support this has been done in terraform provider PR 2725

Jira: NR-296277