Regular expression support for record_transformer's keep_keys and remove_keys

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**Is your feature request related to a problem? Please describe.**

I have a use-case where I am trying to filter out thousands of fields from log records. The fields-to-be-discarded have a fairly predictable name pattern, e.g., `my-field-$N`.

**Describe the solution you'd like**

Add regular expression support to record_transformer's keep_keys and remove_keys.

Following the example above, one could use remove keys as such:

```
remove_keys /^my-field-[0-9]+$/
```

**Describe alternatives you've considered**

In this case, the fields I wanted to retain numbered around 150, so I ended up using keep_keys instead.

**Additional context**

N/A