core doesn't use Record<string, unknown> for public API

[mshustov]

Summary

after TS upgrade some of our public contracts limit end-users to specify type for the passed objects.
example from Security migration
Reason is this change https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#-k-string-unknown--is-no-longer-a-wildcard-assignment-target
What does it mean for us? we should avoid using Record<string, unknown> as a public contract

// before TS update
await clusterClient.callAsInternalUser('endpoint', {headers: ... }); // fine

interface MyPluginParams {
  headers: Headers;
}
const params: MyPluginParams = { headers: ... }
await clusterClient.callAsInternalUser('endpoint', {headers: {}}); // fine

// after TS update
await clusterClient.callAsInternalUser('endpoint', {headers: ... }); // fine

interface MyPluginParams {
  headers: Headers;
}
const params: MyPluginParams = { headers: ... }
await clusterClient.callAsInternalUser('endpoint', {headers: {}}); // error
// Argument of type 'MyPluginParams' is not assignable to parameter of type 'Record<string, unknown>'.
// Index signature is missing in type 'MyPluginParams'

related #40846

Checklist

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• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• [ ] Unit or functional tests were updated or added to match the most common scenarios
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For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

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