Refactor StackSelector for public interface

[mrgrain]

Within the new programmatic toolkit, StackSelector will become part of the public interface. However the current API and implementation is not ideal.

Goal

StackSelector and all related options should adhere to the interface as defined here:

aws-cdk/packages/@aws-cdk/toolkit/lib/types.ts

Lines 62 to 78 in 3b162fc

	export interface StackSelector {
	/**
	* A list of patterns to match the stack hierarchical ids
	*/
	patterns: string[];
	/**
	* Extend the selection to upstream/downstream stacks
	* @default ExtendedStackSelection.None only select the specified stacks.
	*/
	extend?: ExtendedStackSelection;
	/**
	* The behavior if if no selectors are provided.
	*/
	strategy: StackSelectionStrategy;
	}

Steps

1. In CloudAssembly, merge StackSelector with SelectStacksOptions so that selectStacks() only takes a StackSelector.
2. In existing CdkToolkit, change all selectStacksFor* helper methods to take a single stack selector only with now other options. In other words, unify the syntax to always take a CloudAssembly and StackSelector. Merge other options into the stack selector.
3. In CdkToolkit for actions synth, diff, deploy, watch & destroy - merge the exclusively flag into StackSelector because that is the only place it is used.
4. Start reducing the now extended options:

• exclusively should map directly to ExtendedStackSelection.None
• autoValidate ???
• ignoreNoStacks should map to a StackSelectionStrategy.None