Added optional iconName attribute

[bluebeag]

Added optional iconName attribute that allows for icons not matching object name

html:
<c-record-header record-name={name} object-api-name={objectApiName} icon-name={iconNameNotEqualToObjectName}></c-record-header>

js:

 get iconNameNotEqualToObjectName() {
    return "asset_object";
  }

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @bluebeag to sign the Salesforce Inc. Contributor License Agreement.