(DOCSP-35298): C++: Remove BSON from Function examples

[dacharyc]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-35186

• Call a Function: Update examples to remove BSON
• Custom User Data: Update examples to remove BSON
• Authentication/Custom Function User: Update example to remove BSON

Review Guidelines

REVIEWING.md

[github-actions]

Readability for Commit Hash: 6a33e5c

You can see any previous Readability scores (if they exist) by looking
at the comment's history.

Readability scores for changed documents:

• source/sdk/cpp/app-services/call-a-function: Grade Level: 8.7, Reading Ease: 62.88

For Grade Level, aim for 8 or below.

For Reading Ease scores, aim for 60 or above:

Score	Difficulty
90-100	Very Easy
80-89	Easy
70-79	Fairly Easy
60-69	Medium
50-59	Fairly Hard
30-49	Hard
0-29	Very Hard

For help improving readability, try Hemingway App.

[cbullinger]

lgtm!