Implement SHOW FUNCTIONS

[findepi]

Which issue does this PR close?

Closes #12144

Rationale for this change

see #12144

I as a user would want to see a list of available functions

What changes are included in this PR?

Support for Statement::ShowFunctions by converting it directly to VALUES.
Filters are not supported. The syntax for filters is subject to sqlparser-rs/sqlparser-rs#1399

Are these changes tested?

Yes

Are there any user-facing changes?

Support for the SHOW FUNCTIONS statement.

[comphead]

thats a real nice initiative.
I would like to add tests to information_schema.slt like show function
Also we need to expose it to user documentation

[matthewmturner]

It would be really nice if this could somehow include function documentation / signature. ive been wondering how i could get functions and their documentation into datafusion-tui. This is certainly a step in that direction.

[comphead]

are we planning to apply the filter in future?

For example for SHOW now it is possible to display specific parameter like

show datafusion.execution.batch_size

I think it would be interesting to show the function by filter so ther user can get the name and ideally the signature.

Later we can use this metadata table to obtain correct signature in unified way instead of hardcoding signatures like now

[findepi]

that is a good idea.
however, i think sqlparser-rs has the syntax potentialy mixed up.
the filter syntax is borrowed from MySQL while the statement itself isn't borrowed from there, creating a unique mix. see sqlparser-rs/sqlparser-rs#1399
it would be best to resolve this (either fix or close the issue) before jumping on the filtering support

[findepi]

Thank you for the feedback!

In the related issue there is a discussion whether we want SHOW FUNCTIONS syntax at all.
For the time being, I am putting this PR into draft mode to reflect that we first need to have a decision whether and what we want here.

see #12144