[RUM-5418] Add RN API to append to existing user info

[mateo-villa]

What does this PR do?

Adds an addUserExtraInfo API to append user info to an existing user.

Motivation

RUM-5418

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[marco-saia-datadog]

Thank you for the PR! It looks good to me.

The only thing missing is adding addUserExtraInfo to the DdSdkReactNative mock. It is currently making the build fail.

FAIL packages/core/src/__tests__/mock.test.ts
  ● official mock › mocks module: { DdSdkReactNative: [Function] } › for key: addUserExtraInfo
    expect(received).not.toBeUndefined()
    Received: undefined
      73 |                 'for key: %s',
      74 |                 key => {
    > 75 |                     expect(mock[moduleName][key]).not.toBeUndefined();
         |                                                       ^
      76 |                 }
      77 |             );
      78 |         }
      at toBeUndefined (src/__tests__/mock.test.ts:75:55)