class AddChild

Adds a component to the realm.

The `url` can be in one of the following formats:

- an absolute component URL (such as,

"fuchsia-pkg://fuchsia.com/some-package#meta/some-component.cm")

- a relative path component URL to a subpackaged component (like,

"some-subpackage#meta/subpackaged-component.cm"); or

- a relative fragment-only URL (like, `#meta/other-component.cm`;

see https://fuchsia.dev/fuchsia-src/reference/components/url#relative-fragment-only)

indicating the path to a component manifest in the `pkg_dir` supplied

to `RealmBuilderFactory::Create*()`.

The realm builder client typically passes a handle to its own `/pkg`

directory, and bundles the `other-component` into the same package.

Of the three URL types, _only_ component declarations added via the

fragment-only URL can be modified, via

`GetComponentDecl`/`ReplaceComponentDecl` because realm builder actually

performs its own component resolution from the component's manifest and

files bundled in the provided `pkg_dir` (typically from the package

shared by the realm builder client component and the components added

via fragment-only URL).

Errors:

- `CHILD_ALREADY_EXISTS`: this realm already contains a child with the

given name.

- `INVALID_MANIFEST_EXTENSION`: `url` does not end with `.cm`

- `DECL_NOT_FOUND`: The test package does not contain the component

declaration referenced by a fragment-only component URL.

- `DECL_READ_ERROR`: Encountered an I/O error when attempting to read a

component declaration referenced by a fragment-only component URL from

the test package.

- `BUILD_ALREADY_CALLED`: The `Builder.Build` function has been called

for this realm, and thus this `Realm` channel can no longer be used.