feat: support custom action provider

[0xRAG]

What changed? Why?

Adds ability for devs to easily add a custom action provider:

    const custom = customActionProvider<EvmWalletProvider>({
      name: "custom_action",
      description: "This action does XYZ cool thing",
      schema: z.object({
        arg1: z.string(),
        arg2: z.string(),
      }),
      invoke: async (walletProvider, args: any) => {
        const { arg1, arg2 } = args;
        const address = walletProvider.getAddress();
        return `Invoked with ${arg1} and ${arg2} from ${address}`;
      },
    });

    const agentKit = await AgentKit.from({
      walletProvider,
      actionProviders: [custom],
    });