Get-AzMarketplaceTerms should use agreements endpoint

## Description

Get-AzMarketplaceTerms

Get-AzMarketplaceTerms is using the wrong REST API.

- It [is using ](https://docs.microsoft.com/en-us/rest/api/marketplaceordering/marketplaceagreements/get) Microsoft.MarketplaceOrdering/offerTypes/virtualmachine/publishers
- but [should be using ](https://docs.microsoft.com/en-us/rest/api/marketplaceordering/marketplaceagreements/getagreement) Microsoft.MarketplaceOrdering/agreements

A related az cli issue I just opened is https://github.com/Azure/azure-cli/issues/16343.

## Steps to reproduce

```powershell
Get-AzMarketplaceTerms -Publisher artista-networks -Product veos-router -Name eso-4_22_1fx -Debug
```

## Environment data

```
$PSVersionTable

Name Value
---- -----
PSVersion 7.1.0
PSEdition Core
GitCommitId 7.1.0
OS Microsoft Windows 10.0.19042
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0
```

## Module versions
It is the latest version, Az.MarketplaceOrdering 1.0.2.
https://www.powershellgallery.com/packages/Az.MarketplaceOrdering/1.0.2

```powershell
get-module -Name Az.MarketplaceOrdering | fl

Name : Az.MarketplaceOrdering
Path : C:\Users\cataggar\Documents\PowerShell\Modules\Az.MarketplaceOrdering\1.0.2\Az.MarketplaceOrdering.psm1
Description : Microsoft Azure PowerShell - Marketplace Ordering agreements service cmdlets for Azure Resource Manager in Windows PowerShell and PowerShell Core.

For more information on Marketplace Ordering, please visit the following: https://docs.microsoft.com/azure/marketplace/
ModuleType : Script
Version : 1.0.2
PreRelease :
NestedModules : {Microsoft.Azure.PowerShell.Cmdlets.MarketplaceOrdering}
ExportedFunctions :
ExportedCmdlets : {Get-AzMarketplaceTerms, Set-AzMarketplaceTerms}
ExportedVariables :
ExportedAliases :
```

## Debug output

```

```

## Error output

```

```

Start at the Get-AzMarketplaceTerms cmdlet in Az.MarketplaceOrdering and compare its request with the linked MarketplaceOrdering API documentation. Reproduce the issue with the provided PowerShell command, then verify that the cmdlet uses Microsoft.MarketplaceOrdering/agreements instead of the offerTypes endpoint. Done means the cmdlet retrieves the expected marketplace agreement.

Written by the indexing model from the issue text.