feat(core): Add connection to optional remote ers

[elizabethhealy]

Adding modeoptions to config

• allows user to specify certain remoteERSUrl if the ERS is running remotely. can be extended to support more options

Support a separate ers connection in the sdk

• i thought about have two clients, one for the core platform and one for ers, but then we'd need each the opentdfserver to store multiple sdk clients and the service registration functions to take in multiple sdk clients, it felt like a more invasive change, but idk maybe that's cleaner?

[elizabethhealy]

TODO: support dial options in the yaml config

[github-actions]

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[strantalis]

What are your thoughts if we moved this configuration underneath SDKConfig. Trying to think how we could layer in other options for our internal products as well.

platform/service/internal/config/config.go

Lines 45 to 58 in c36624c

	type SDKConfig struct {
	// Endpoint is the URL of the Core Platform endpoint.
	Endpoint string `mapstructure:"endpoint" json:"endpoint"`
	// Plaintext specifies whether the SDK should use plaintext communication.
	Plaintext bool `mapstructure:"plaintext" json:"plaintext" default:"false" validate:"boolean"`
	// ClientID is the client ID used for client credentials grant.
	// It is required together with ClientSecret.
	ClientID string `mapstructure:"client_id" json:"client_id" validate:"required_with=ClientSecret"`
	// ClientSecret is the client secret used for client credentials grant.
	// It is required together with ClientID.
	ClientSecret string `mapstructure:"client_secret" json:"client_secret" validate:"required_with=ClientID"`