[SidePanel] Consolidate SidePanelEntry constructors

This CL consolidates the 2 different SidePanelEntry constructors into
one for convenience and the other being the primary constructor.

The reason there was 2 separate ones is because extensions doesn't use
any of the callbacks the larger constructor uses. Overtime this allowed
the constructors to diverge.

To consolidate we remove use of optional callbacks in the constructor
relying on callers to either provide an implementation or pass
base::NullCallback.

The extensions constructor is then converted into a convenience
constructor that is used for all entries that do not rely on the
additional callbacks.

Change-Id: Ic930e2d411338506a5e793f1ca6067c67185b25e
Bug: 416585199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6528589
Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Darryl James <dljames@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459508}