[NV-991] Test Mandrill provider connection when creating/updating integration

[BiswaViraj]

Why?

Currently, when the user connects a provider from the Integration store, there's no option to Test the provider connection before saving/updating it for wrong/bad credentials or other possible issues.

Without the Test functionality, the provider connection is saved with the possible wrong credentials and the issue is discovered at a much later stage.

We want to have a Test functionality during the connection stage to let users catch the errors as early as possible.

How?

We will need to create a new method i.e,checkIntegration for the Mandrill provider.

Currently, we have a placeholder checkIntegration method, you will need to update this method accordingly.

Path to the file : providers/mandrill/src/lib/mandrill.provider.ts

Look into the Mandrill docs and if they provide a dedicated endpoint or method to test the connection, then implement the same inside the checkIntegration method.

If there's no dedicated endpoint/method provided by Mandrill, then re-use the send functionality of the provider to check and ensure proper integration, and return the errors if any.

P.S: We have integrated the same for the sendgrid provider, you may use it as a reference or follow this pr for more details.

_{From SyncLinear.com | NV-991}

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