Add query to vendor

[heimweh]

This PR adds support for querying vendors.

This allows us to easier find vendors by name.

Example:

client.ListVendors(&pagerduty.ListVendorOptions{
    Query: "Datadog",
}

Tested and seems to work quite well.

[heimweh]

Would be awesome to be able to query vendors by name like this.
Would any of you mind taking a look? :)

I'm not 100% sure if there are other options that should be in there as well.

//cc @ranjib @Deconstrained

[lfepp]

LGTM