Convert adapter utils to middlewares

[marefr]

What this PR does / why we need it:
Follow up from #1117 where I decided to put the convert adapter utils to middlewares in a separate PR. This converts common adapter logic to middlewares that can be reused between the SDK and Grafana.

Which issue(s) this PR fixes:
Ref #1101
Ref #1117

Special notes for your reviewer:

[andresmgot]

sounds good, a little to big to give a more meaningful review. I added some questions

[andresmgot]

this was a bit confusing since the "tracingWrapper" got moved to a middleware in this PR but the rest aren't. I just saw that NewMetricsMiddleware, etc already exist.

[marefr]

Yes it was added in #1117 as explained in the description. It was added so Grafana could start use it sooner rather than later given this is quite a lot of changes

[andresmgot]

any reason for not doing this propagateTenantIDIfPresent as a middleware too?

[marefr]

This is running before protobuf request converted to SDK request. @wbrowne you see any risk changing this to middleware?

[wbrowne]

Nothing should be depending on this other than the instance providers (and it looks like the tracer), so it should be fine I think

[andresmgot]

is there anything setting this now?

[marefr]

Yes, this

grafana-plugin-sdk-go/backend/handler_middleware.go

Lines 49 to 57 in abcbdd9

	func (h *MiddlewareHandler) setupContext(ctx context.Context, pluginCtx PluginContext, endpoint Endpoint) context.Context {
	ctx = initErrorSource(ctx)
	ctx = WithEndpoint(ctx, endpoint)
	ctx = WithPluginContext(ctx, pluginCtx)
	ctx = WithGrafanaConfig(ctx, pluginCtx.GrafanaConfig)
	ctx = WithUser(ctx, pluginCtx.User)
	ctx = WithUserAgent(ctx, pluginCtx.UserAgent)
	return ctx
	}