aws-lambda: `any` return types don't agree with strict ESLint rules

### Describe the bug

In some places `aws-cdk` returns `any` but this causes trouble for projects that forbid usage of `any` (e.g. via ESLint rules).

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

Returns types are properly typed so as to not trigger any `any` assignment lint warnings/errors.

### Current Behavior

Some APIs return `any`.

### Reproduction Steps

```ts
lambda.addEventSource(
new DynamoEventSource(table, {
startingPosition: StartingPosition.LATEST,
filters: [
FilterCriteria.filter({
eventName: FilterRule.isEqual('EVENT_NAME'),
userIdentity: {
type: FilterRule.isEqual('TYPE'),
principalId: FilterRule.isEqual('PRINCIPAL_ID'),
},
}),
],
}),
);
```

the usages of `FilterRule.isEqual()` are flagged with the following error:

> Unsafe assignment of an `any` value. eslint[@typescript-eslint/no-unsafe-assignment](https://typescript-eslint.io/rules/no-unsafe-assignment)

### Possible Solution

Change the following in event-source-filter.d.ts from...

```ts
static isEqual(item: string | number | boolean): any;
```

to...

```ts
static isEqual(item: string): [string];
static isEqual(item: number): [{ numeric: ['=', number] }];
static isEqual(item: boolean): [boolean];
```

### Additional Information/Context

_No response_

### CDK CLI Version

2.161.0 (build be5ad8b)

### Framework Version

_No response_

### Node.js Version

v20.18.0

### OS

macOS Sonoma 14.7.1 (23H222)

### Language

TypeScript

### Language Version

5.6.2

### Other information

_No response_

Start with the `event-source-filter.d.ts` declaration mentioned in the issue and inspect the return type of `FilterRule.isEqual()`. Reproduce the reported unsafe-assignment warning in the provided `DynamoEventSource` example. Done when the return types no longer cause the reported lint error; the issue has had no comment for over 180 days.

Written by the indexing model from the issue text.