.NET: Review HostApplicationBuilderAgentExtensions for consistency with other Agent registration methods

HostApplicationBuilderAgentExtensions has a number of methods to register ChatClientAgent instances, but the signatures of these do not match other parts of the code base, like creating an agent from an SDK.

In other parts of the code there are two overloads with two standard parameter sets:

With separate params:
string? instructions = null,
string? name = null,
string? description = null,
IList? tools = null,

With options:
ChatClientAgentOptions? options,

We should review these and consider making these consistent