Update format of new Changelog Entry (#1633)

Update changelog entry to match updated guidance https://azure.github.io/azure-sdk/policies_releases.html#change-logs

New Changelog entries will now look like
```
# Release History

## 1.15.0 (Unreleased)

### Features Added

### Breaking Changes

### Key Bugs Fixed

### Fixed


## 1.15.0-beta.1 (Unreleased)

### Features Added

- Types to represent `GeoJson` primitives.
```

eng/common/scripts/ChangeLog-Operations.ps1

@@ -40,19 +40,32 @@ function Get-ChangeLogEntriesFromContent {
   $changeLogEntries = [Ordered]@{}
   try {
     # walk the document, finding where the version specifiers are and creating lists
-    $changeLogEntry = $null
     foreach ($line in $changeLogContent) {
       if ($line -match $RELEASE_TITLE_REGEX) {
         $changeLogEntry = [pscustomobject]@{ 
           ReleaseVersion = $matches["version"]
           ReleaseStatus  =  $matches["releaseStatus"]
           ReleaseTitle   = "## {0} {1}" -f $matches["version"], $matches["releaseStatus"]
           ReleaseContent = @()
+          Sections = @{}
         }
         $changeLogEntries[$changeLogEntry.ReleaseVersion] = $changeLogEntry
       }
       else {
         if ($changeLogEntry) {
+          if ($line.Trim() -match "^###\s(?<sectionName>.*)")
+          {
+            $sectionName = $matches["sectionName"].Trim()
+            $changeLogEntry.Sections[$sectionName] = @()
+            $changeLogEntry.ReleaseContent += $line
+            continue
+          }
+
+          if ($sectionName)
+          {
+            $changeLogEntry.Sections[$sectionName] += $line
+          }
+
           $changeLogEntry.ReleaseContent += $line
         }
       }
@@ -195,7 +208,19 @@ function New-ChangeLogEntry {
     return $null
   }
 
-  if (!$Content) { $Content = @() }
+  if (!$Content) { 
+    $Content = @()
+    $Content += ""
+    $Content += "### Features Added"
+    $Content += ""
+    $Content += "### Breaking Changes"
+    $Content += ""
+    $Content += "### Key Bugs Fixed"
+    $Content += ""
+    $Content += "### Fixed"
+    $Content += ""
+    $Content += ""
+  }
 
   $newChangeLogEntry = [pscustomobject]@{ 
     ReleaseVersion = $Version