Az.SecurityInsights create Cmdlets for Watchlist management

## Description of the new feature

Managing Azure Sentinel from Powershell implies possibility to operate on all items provided by solution.
A key element is watchlist; what can be managed [directly _via_ REST API](https://docs.microsoft.com/en-us/azure/sentinel/watchlist-with-rest-api) already.

New Cmdlets could wrap all API endpoints for watchlists.

## Proposed implementation details (optional)

**`New-AzSentinelWatchlist`** to create watchlist
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`, `rawCSVContent`, `content`
- output : `Boolean` (action succeeded)

**`Get-AzSentinelWatchlist`** to fetch watchlist
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`
- output : `Microsoft.Azure.Commands.SecurityInsights.Models.Watchlist.PSSentinelWatchlist`

**`Remove-AzSentinelWatchlist`** to remove watchlist
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`
- output : `Boolean` (action succeeded)

**`Update-AzSentinelWatchlist`** to update watchlist as a whole
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`, `content`
- output : `Boolean` (action succeeded)

**`Update-AzSentinelWatchlistItem`** to update watchlist item
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`, `watchlistItemId`, `value`
- output : `Boolean` (action succeeded)

**`Add-AzSentinelWatchlistItem`** to add watchlist item to existing Watchlist
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`, `watchlistItemId`, `value`
- output : `Boolean` (action succeeded)

**`Remove-AzSentinelWatchlistItem`** to remove watchlist item to existing Watchlist
- param : `watchlistAlias`, `resourceGroup`, `subscriptionId`, `workspaceName`, `watchlistItemId`
- output : `Boolean` (action succeeded)

Where:
- `content` as `PSSentinelWatchlist`, every other param as String.

Resources: https://docs.microsoft.com/en-us/answers/questions/518881/is-it-possible-to-manipulate-azure-sentinel-watchl.html

I cen help to contribute... Even though I'm newbie to azure-powershell.

Start by reviewing the linked Azure Sentinel watchlist REST API and the existing Az.SecurityInsights module entry points. Compare the proposed New, Get, Remove, Update, Add, and item-management cmdlets with the API endpoints, parameter types, and requested outputs; done means the watchlist operations are implemented and covered by the module's tests.

Written by the indexing model from the issue text.