refactor!: Improve Tool abstractions

[davidmigloz]

Preparation work for #397

Changes

• ToolSpec: new class that contains the specification of a LangChain tool without the actual implementation.
• BaseTool has been renamed to Tool: the base class for a tool (implements ToolSpec). It also now has 3 generic parameters: the input type, the options type and the output type.
• Classes that extend from Tool must now implement a getInputFromJson method that converts a JSON Map into the input type of the tool (used to convert the response from the language model to the actual input type).
• Tool has been renamed to StringTool, tools that accept a single string input and returns a string output.

Example of a String tool:

final echoTool = StringTool.fromFunction(
  name: 'echo',
  description: 'echo',
  func: (String input) => input,
);

Example of a tool with a custom input object:

class SearchInput {
  const SearchInput({
    required this.query,
    required this.n,
  });

  final String query;
  final int n;

  SearchInput.fromJson(final Map<String, dynamic> json)
      : this(
          query: json['query'] as String,
          n: json['n'] as int,
        );
}

String callYourSearchFunction(final SearchInput input) {
  return 'Results:\n${List<String>.generate(input.n, (final i) => 'Result ${i + 1}').join('\n')}';
}

final tool = Tool.fromFunction<SearchInput, String>(
  name: 'search',
  description: 'Tool for searching the web.',
  inputJsonSchema: const {
    'type': 'object',
    'properties': {
      'query': {
        'type': 'string',
        'description': 'The query to search for',
      },
      'n': {
        'type': 'number',
        'description': 'The number of results to return',
      },
    },
    'required': ['query'],
  },
  func: callYourSearchFunction,
  getInputFromJson: SearchInput.fromJson,
);