feat: update Discover flow to support update resource mechanism

[flyinghermit]

Updates DiscoverComponent component and useDiscover to support update resource mechanism.

A new props updateFlow have been added of type DiscoverUpdateProps:

export type DiscoverUpdateProps = {
  // resourceSpecForUpdate specifies ResourceSpec which should be used to
  // start a Discover flow.
  resourceSpec: ResourceSpec;
  // agentMetaForUpdate includes data that will be used to prepopulate input fields
  // in the respective Discover compnents.
  agentMeta: AgentMeta;
};

Implemented to support SAML application update flow - https://github.com/gravitational/teleport.e/issues/4458

[flyinghermit]

Not entirely sure if this (queryAllByTestId().length).toBeFalsy()) is the correct way to check that resources are not rendered for the update flow. lmk if theres a better way.

[gzdunek]

Maybe:

expect(screen.queryByTestId((ResourceKind.Server)).not.toBeInTheDocument();

[gzdunek]

Maybe:

expect(screen.queryByTestId((ResourceKind.Server)).not.toBeInTheDocument();

[flyinghermit]

Friendly ping.

[flyinghermit]

As this one is pending review, I am closing it and I instead I will add these changes to the followup #43871 which depends on this one and implements the changes required for supporting SAMl app update flow as a whole.