Embed: Add ability to navigate a project's dashboards

[ericpgreen2]

Closes: https://github.com/rilldata/rill-private-issues/issues/211

The following examples show various requests to the GetIFrameURL API described in the docs.

To embed a single dashboard:

const response = await fetch(`https://admin.rilldata.com/v1/organizations/${rillOrg}/projects/${rillProject}/iframe`, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    Authorization: `Bearer ${rillServiceToken}`,
  },
  body: JSON.stringify({
    // simply provide a resource
    resource: 'dashboardName'
  }),
});

To embed a dashboard and enable navigation between dashboards:

const response = await fetch(`https://admin.rilldata.com/v1/organizations/${rillOrg}/projects/${rillProject}/iframe`, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    Authorization: `Bearer ${rillServiceToken}`,
  },
  body: JSON.stringify({
    // enable navigation and provide a resource
    navigation: true,
    resource: 'dashboardName'
  }),
});

To embed the dashboard list, which enables navigation to any of the project's dashboards:

const response = await fetch(`https://admin.rilldata.com/v1/organizations/${rillOrg}/projects/${rillProject}/iframe`, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    Authorization: `Bearer ${rillServiceToken}`,
  },
  body: JSON.stringify({
    // enable navigation and do not provide a resource
    navigation: true
  }),
});