Add User Agent Header

[Cliftonz]

Add the user-agent request header to every call in this sdk.

Example:

headers: {
Authorization: Bearer ${ApiKey},
"User-Agent": novu/${sdk}@${version},
},

Why? (Context)

Having request headers in the SDKs will help us in monitoring which SDK is mostly used by the community and customers. It will help us make the decisions in the future on which SDKs to officially support in the future.