…lastic#55982)

When merging component template, index template, and request mappings, we now treat any declaration
of a top-level field the same as replacing all sub-objects. For example, assuming two component
templates with mappings and template B taking precedence:

```
A: {foo: {...}}
B: {foo.bar: {...}}
Result: {foo.bar: {...}}

A: {foo.bar: {...}}
B: {foo: {...}}
Result: {foo: {...}}

A: {foo.bar: {...}}
B: {foo.baz: {...}}
Result: {foo.baz: {...}}
```

Relates to elastic#53101