[KeyVault] Add Support for Private Link for Key Vault

[dingmeng-xue]

Description of the new feature

Add support for private link for KeyVault

Relevant Swagger change is Azure/azure-rest-api-specs#7631

Proposed implementation details (optional)

(Updated 4/8/2020)

• PowerShell will centralize all private link relating cmdlets in Az.Network module
• These cmdlets are generic, not reousrce-type-specific
• Scenario tests will be in each modules
• The approve operation will be designed as Approve-AzPrivateEndpointConnection

More info

• The top level resource of keyvault is "vaults"
• We'll use API version 2018-02-14

[isra-fel]

An open question / decision to make:

Do we want to design the approve/reject private endpoint connection as

1. commands, e.g. Approve-AzKeyVaultPrivateEndpointConnection
2. parameter in Update-* command, e.g. Update-AzKeyVaultPrivateEndpointConnection -Approved $True

Cli had same discussion here: https://github.com/Azure/azure-cli/pull/12073/files/b2d332d4e2320afd4d8bb717a4a530245f8f5f2a#r377498807

Cli PR: Azure/azure-cli/pull/12073
They used commands, but it was not under fully discussion

[bim-msft]

An open question / decision to make:

Do we want to design the approve/reject private endpoint connection as

1. commands, e.g. Approve-AzKeyVaultPrivateEndpointConnection
2. parameter in Update-* command, e.g. Update-AzKeyVaultPrivateEndpointConnection -Approved $True

Cli had same discussion here: https://github.com/Azure/azure-cli/pull/12073/files/b2d332d4e2320afd4d8bb717a4a530245f8f5f2a#r377498807

Cli PR: Azure/azure-cli/pull/12073
They used commands, but it was not under fully discussion

@isra-fel Hi, we plan to send out an email involving you guys and Key Vault team to further discuss this open question.