feat(transfer): add vpcendpoint dns entries to connection secrets

[anagarlau]

Description of your changes

Fixes #

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

[MisterMX]

I think we can avoid a second GET on the providerconfig by implementing a custom connector that returns a custom external with the standard external nested in it. Something like:

type customConnector struct {
	kube client.Client
	opts []option
}

func (c *connector) Connect(ctx context.Context, mg cpresource.Managed) (managed.ExternalClient, error) {
	// the code can either be copied from zz_controller.go or reused and the result casted (whatever feels better)

        cr, ok := mg.(*svcapitypes.Server)
	if !ok {
		return nil, errors.New(errUnexpectedObject)
	}
	sess, err := connectaws.GetConfigV1(ctx, c.kube, mg, cr.Spec.ForProvider.Region)
	if err != nil {
		return nil, errors.Wrap(err, errCreateSession)
	}

	ec2Client := c.newClientFn(*cfg)
	if vpcClient == nil {
		return nil, errors.New("failed to initialize EC2 client")
	}

	return &customExternal{
		ec2: ec2Client
		external: newExternal(c.kube, svcapi.New(sess), c.opts)
	}
}

type customExternal struct {
  *external
  ec2Client ec2.Client
}

func (e *customExternal) postObserve() {
  // endpoint observation + existing code
}

[anagarlau]

Technically this would be the best way to do it but given that the Connect method on the connector is already generated, this approach would require generated code to be removed. I m not sure it can be skipped. However, I could not call the Connect twice but implement the functionality in (c *connector) Connect in the (c *customConnector) Connect and remove any redundancies.
Also, ec2 Client needs the v2 Config while the transfer client uses v1

[MisterMX]

LGTM. Thank you very much @anagarlau! Since this is purely cosmetic addition and before that transfer was pretty unusable without the endpoint I guess we can backport this to v0.51.

[github-actions]

Successfully created backport PR #2147 for release-0.51.