xds: change `ServerConfig` type to have a single method to return all configured dial options

[easwars]

The ServerConfig type in internal bootstrap package currently stores the credentials configured for the server config as a dial option here: https://github.com/grpc/grpc-go/blob/master/internal/xds/bootstrap/bootstrap.go#L177-L181 and it has a method to return the configured credentials as dial option here:

grpc-go/internal/xds/bootstrap/bootstrap.go

Line 222 in 8ea3460

func (sc *ServerConfig) CredsDialOption() grpc.DialOption {

As part of #7586, we are adding more dial options to the server config, and the PR currently adds a new method to return the new dial option. Once that PR is merged, we should instead have a single method in the ServerConfig type that returns a slice of dial options.

These dial options are usually retrieved from the ServerConfig when a transport to the corresponding server is being created. This is currently where it is being used:

grpc-go/xds/internal/xdsclient/transport/transport.go

Line 197 in 8ea3460

opts.ServerCfg.CredsDialOption(),

The caller here does not care what the dial options are, and instead of making multiple calls to retrieve all the configured dial options, it would be easier/nicer to make a single call on the ServerConfig that returns all configured dial options.