Timechart should only allow for one by clause

[51-code]

Describe the bug

index=abc earliest =-4y | timechart count by source avg(host) by sourcetype
works, even though only one BY clause should be possible.

Expected behavior

A list of aggregates can be given, but there should only be one by clause for all of them.

Example: index=abc earliest =-4y | timechart count avg(host) by sourcetype
where both count and avg(host) will be grouped by the sourcetype column.

How to reproduce

Run the query above.

Screenshots

Software version

5.4.0

Desktop (please complete the following information if relevant):

• OS:
• Browser:
• Version:

Additional context

[51-code]

Internal PR created.

However, not going to merge until the relevant changes are ready in PTH-10.