Add experimental test manage func

[wbrowne]

What this PR does / why we need it:
Adds a new (experimental) API for plugins to create a running backend for test purposes.

Usage:

const addr = "127.0.0.1:8000"

factoryInvocations := 0
factory := datasource.InstanceFactoryFunc(func(settings backend.DataSourceInstanceSettings) (instancemgmt.Instance, error) {
	factoryInvocations++
	i, err := NewDatasource(settings) // plugin's factory func
	return i, err
})

// <newAPI>
tp, err := datasourcetest.ManageForTest(factory, datasourcetest.ManageOpts{Address: addr})
require.NoError(t, err)
defer func() {
	err = tp.Client.Shutdown()
	t.Log("client shutdown error", err)
	err = tp.Server.Shutdown()
	t.Log("server shutdown error", err)
}()
// </newAPI>

pCtx := backend.PluginContext{DataSourceInstanceSettings: &backend.DataSourceInstanceSettings{
	ID: 1,
	JSONData: []byte(`{"foo":"bar"}`),
	DecryptedSecureJSONData: map[string]string{"secret": "p@ssw0rd",},
}}

t.Run("Test something", func(t *testing.T) {
	resp, err := tp.Client.QueryData(context.Background(), &backend.QueryDataRequest{PluginContext: pCtx})
	require.NoError(t, err)
	// ...

Special notes for your reviewer:

[andresmgot]

Sounds good to me

[andresmgot]

I think you added my commits here by mistake 😅

[wbrowne]

I think you added my commits here by mistake 😅

Doh! Thanks for catching - should be fixed 😄

[andresmgot]

LGTM 👍

[xnyo]

Awesome work!! 🚀 🚀

Just one small suggestion:

[xnyo]

LGTM, great work!! 🚀🚀